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INSTRUCTION MANUAL
MODEL 515
MEGOHM BRIDGE
WARRANTY
We warrant each of our products to be free
from defects in material and workmanship. Our
obligation under this warranty is to repair or
replace any instrument or part thereof which,
within a year after shipment, proves defective
upon examination. We will pay domestic
surface freight costs.
To exercise this warranty, call your local
field representative or the Cleveland factory,
DDD 216-248-0400. You will be given assist-
ance and shipping instructions.
REPAIRS AND RECALIBRATION
Keithley Instruments maintains a complete re-
pair service and standards laboratory in Cleve-
land, and has an authorized field repair facility
in Los Angeles and in all countries outside the
United States having Keithley field repre-
sentatives.
To insure prompt repair or recalibration serv-
ice, please contact your local field representa-
tive or the plant directly before returning the
instrument.
Estimates for repairs, normal recalibrations,
and calibrations traceable to the National Bu-
reau of Standards are available upon request.
MODEL 515 MEGOHMBRIDGE CONTENTS
TABLE OF CONTENTS
Section Page Section Page
I. INTRODUCTION . . . . . . . . . I-l 8. Time Constants - Slow
Response . . . . . III-13
II. SPECIFICATIONS . . . . . . . II-1 9. Transients Caused by
Push-to-Read Switch. III-14
III. OPERATION. . . . . . . . . . . III-1 10. Verification of the
Accuracy . . . . . III-14
A. Outline of Procedure . . . 111-1
B. Description of Controls IV. CIRCUIT DESCRIPTION . . . . . IV-1
and Terminals . . . . . , III-2
C. Operation Steps. . . . . . . III-3 A. power Supply. . . . . IV-1
D. Standardization. . . . . . . III-4 B. Null Detector . . . . . IV-1
E. Connecting the Unknown C. Bridge Circuitry. . IV-3
Resistor. . . . . : III-4 D. Overvoltage Protection. . IV-4
F. External Bridge Voltage
Supply. . , , . . . . . , III-8 V. MAINTENANCE . . . V-l
G. Voltage Across Unknown and
Standard Resistors. , , , . III-8 6. REPLACEABLE PARTS . . . . . 6-l
H. Accuracy Considerations. . . III-9
1. Null Detector 6-l. Replaceable Parts List. 6-l
Sensitivity . . . . . . III-9 6-2. How to Order Parts. . 6-l
2. Null Detector Zero Model 515 Replaceable
Drift , . . . . . . . . III-10 Parts List . . . . 6-2
3. Resolution of the Model 515 Schematic
Readout . . . . . ; . . III-10 Diagram 14522D . . . 6-10
4. Accuracy of the Read- Green Calibration
out and Standard and Repair Form . . . . 6-11
Resistors . . . . . . . III-10
5. Temperature and Voltage
Coefficients in the * Change Notice Last page
Bridge Resistors. . . . 111-12
6. Leakage Resistance
Across the Unknown. . . III-12 *Yellow Change Notice sheet is included
7. Errors Caused by Guard only for instrument modifications affect-
to Ground Resistance. . 111-12 ing the Instruction Manual.
0568R i
SECTIONI - INTROlXJClTON
The Model 515 MegohmBridge if5a Wheatstone Bridge for measuring
resistors from 105 ohme to 10 ohms with accuracies from 0.05% to
1.0%. It is complete, with an electrometer null detector, shielded
enclosure for the unknown, and a bridge voltage supply.
A unique system of switches is provided to allow corrections to be
made for the slow changes in resistance of the standard high megohm
resistors. This enables all values of resistance to be read with rated
accuracy, directly from the bridge dials.
Bridge voltages in one volt steps up to 10 volts are available from
the internal supply. With external supplies, voltages as high as
1,000 volts csn be used.
A connector is provided so that unknown resistors can be measured
outside the instrument as well as in the built-in shielded enclosure.
SECTIONII - sPFXXF1cATI0NS
RANGE: lO5 to l&5 ohms with a six-dial in-line readout.
ACCURACY:As tabulated below, if bridge is operated so that volt-
age *cross stendard resistor does not exceed 10 volts.
RangeOhms Accuracy Possible Bridge Min. Volt. for
Voltage Rated Accuracy
105 to 10-f 0.05% 1tolooOV 5v
10 to 108 0.05 1tolocn 50
10;I to 109 0.10 1 to loo0 3
109 to 1010 o.l.5 1toloco 2
1010 to loll 0.2 1 to 1000 1
loll to loI2 0.25 1t0lOo0 1
12 to lo13 0.30 1 to 1000
El3 to 1014 0.5 1to 1000 1;
1014 to lo15 1.0 10 to 1000 100
For less than minimum voltage, accuracy decreases in proportion
to the ratio of applied voltage to minimum stated voltage.
INKTT: Built-in compartment or Remote Test Chamber with teflon-
insulated triexial cable.
GROUNDING:One terminal of unknown is at ground potential.
yy4DmmR Electrometer with a grid current of less than 5 x
amperes and sensitivity ranges of lvolt perdiv. to 1 mill-i-
volt per diw in decade ranges. Reading is non-linear past l/3 of
full scale for ease in balancing.
ZEROCREYX Normally closed zero-check button shorts out null de-
tector input except when depressed.
BRIWEFQTERTI&z Internal: From zero to 10 volts in one volt steps
selectable from the front panel. External: With Keithley Model 2443
or 241Power Supply, from zero to 100 volts on any resistance reading,
from 100 to 1000 volts provided the readout dial is at least in the
x10 position. Bridge interlocks, and the inherent overload protection
of the Keithley power supplies, prevent damage if readout dial is in-
advertently placed in the wrong position. Since other types of power
supplies do not provide the correct overload protection, only the
Keithley Model 240 or 241 is recormnended.
POWER: 100-130 or 200-260 volts, 50-60 cps. 10 watts.
TIJBEANDTRANSIS!t.OR i-5886, 2-6418, 1-0~2; 1-2~1535,
COMPLE!MER?
6-21il381.
ACCESSORIES AVAILABLE: Model 5151 End Frames with mounting hardware,
rubber feet. Model 5154 Cabinet; Model 5152 Remote Test Chamber with
60" triaxial cable and bridge connector; Model 5153 60" trisxial. cable
with bridge connector on one end.
515 II - 1
DIMENSIONB: Model 515 bfegolpnBridge, 19" w x 14" h x I@" d.
Model5154 Cabinet, 21" w x 25" h x 16&f a.
NE2 WEIGR!J!z Model 515 Megolm Bridge, 24 lbs. Model. 5154 Cap-
inet, 52 lbs.
515 II - 2
SECTIONIII - OPFRATION
A. OUTLINEOF PROCEDURE, (taken from instructions fastened to the
inside of the door of the Shielded Measuring Compartment).
1. Connect power cord to 115 volts, 50/60 cps unless specified
on rear for 230 volt. To change line voltage see Section V -
Maintenance.
2. Turn on power; set MILLIVOI/IS PER DIVISION switch to 1000;
release HJSH'IO READswitch. Set meter to zero with FINE ZERO.
If necessary use COARSE ZERO. Increase sensitivity and rebal-
ace. Drift which msy be apparent at maxirann sensitivity will
become negligible after a short warm-up.
3. STANDARM- Set FUNC'IIONswitch to STANDARDIZE: MULT-
1PLIERdialtom; and RESISTAEtX, OHMS dials to 10.000. Bring
to exact null with ORMS dials; at the same time increase the null
detector sensitivity to ma%-. Release HJSH READbutton
and set FUNCTION 9
switch to CALIBRATE. Adjust l0 CALIBRATE
potentiometer to give a null when II 9?3FONDswitch is oper-
ated. Next set exponent dial t&IO T and repeat step 3. Do
the same in sequence UJP thrn I.0 . This completes the bridge
standardization.
4. 0PEwJ!I0N: Place resistor to be measured in compartment.
Locate the ground clip to suit the resistor length, and close
compartment.
Select bridge voltage. Internal voltages from 1 to 10 volts
or external voltages up to 100 volts may be used with no special
precautions. Above I.00 volts the xl0 or xUl0 dial must not be
set at zero. With the Kelthley 240 or 241 Power Sypp3y, the
interlock circuit will prevent damage.
Set null detector sensitivity to minimum and operate PUSHM
READbutton. If miU. detector deflects to left the readout
dials (RESISTAECE,OBMS)are set below value of resistance.
Increase the indicated mISTAWCE until a null is obtained.
If null detector deflects to right reduce the indicated RE-
SISTANCE. If no deflection is observed increase null detector
sensitivity. Final balance should be made tith enough sensltiv-
ity to give required accuracy.
For external operation, attach special cable only to input con-
nector and set FUEtX!IONswitch to EiilWNU OPEW. Since the
door interlock is now inoperative observe care with high bridge
voltages.
5. ACCURACY:As tabulated below, if bridge is operated so
that voltage acrose stendard resistor &es not exceed 10 volts.
515 III -1
Rang% ohms Accuracy Possible Bridge Min. Volt. for
Voltage Rated Accuracy
I.05 to lo; 1tolooov 5v
I.07 lo to loo0 50
108 : $9 1to lGQ0 3
I.09 to 1010 1 to loo0 2
lo~tolo~ 1 to loo0 1
lo~tolLG 1tOlOOO
lo12 to I.013 0.36 1 to 1000 ;
lo13 to lo*
lo14 to I.015 0.5
1.0 ltolcoo
lotoKxxl lE
For less then mlnimm voltage, acouracy decreases in proportion
to the ratio of applied voltage to minirmpmstated voltage.
B. DEWRIPl!IONOFCONTROU3AND~AIS:
BRIR3EVOUCS: This rotary switch adjusts the voltage applied
to the bridge In 1 volt steps up to I.0 volts and &so Is used
to energize the external supply circuit when it is in the EK!l!
position.
ON: Toggle switch is the main power switch. Preseme of power
is indicated by the i-ted meter dial.
MIUXVOIfE3 PER DIVJSIOIk Rotary switch provides decade step6
of nuU detector sensitivity.
FINE ZERO: Ten-turn control18 used for sett%ng the null de-
tector to zero.
COARBEZERO: Eleven position rotary switch sets the meter zem
,&thin the range of the FINE ZEROcontrols. It mey be swltchee8
with a screwdriver from the fxont panel.
NUT&INDICAMR: Three-inch meter, Incorporating a non-linear
;mvement for easy bridge balancing,
'RTSH9X) READz Push-button switch mxnNJy shorts the miJ.l de-
tector @put. It may be locked In the open position.
'FUNCTION Four position rotary stitch provides the necessary
circuit arrangements for calibration of the standards, and also
employed when the unknown resistor is outslde the instr7.mvd.
'RESIS'JANCE,OHMS: These seven di.als include five decade step
xw-itches and one rheostat which forn the variable arm of the
bridge. !W seventh dial is a nailtiplier switch. At b&lance,
the unknown resistance is read directly from these dials.
Below these dials is the shielded test chamber. This contains
the external input connector and 81s calibration controls in
addition to the guarded test terminal. The external input con-
nector is a teflon insulated triexial receptacle (Gremw 5632~).
515 III - 2
EUSE: A fuse extractor post Is located on the rear of the in-
strmnent . For ILL5 volt operation use a 3 AG, t amp. fuse; for
230 volts use a 3 b3, l/8 mqp.
POWER CORD: The three wire cord with the NlNA approved three-
prong plug provides a ground connection for the cabinet. An
adapter to allow operation from twn prong outlets is provided.
ACCESSORY OUTLET: A three-terminal convenience outlet Is pro-
vided on the rear for operation of an external power supply.
It is wired directly to the powr cord and is not controlled
by the bridge power stitch.
EXTEBNAL INHTl!z UHF receptacle on the rear of the instrument,
used to connect an external power sqpply when bridge voltages
above 10 volts are desired.
C. OPERATION
STEPS
1. Connect power cord to ll5 volt, g/f50 cps, unless specified
on resx for 230 volts. !Co change line voltage see Section V,
Maintenance.
2. Set null detector to l.CW milklvnlts per cllvislon and un-
lock PUSH'JB READsnitch so null detector input is shorted (note
the null detector is normeSLy shorted corresponding to sn open
galvanonder key).
3. !Cum power on and allow 30 seconds for warm-up. The meter
should indicate zero. Increase mill sensitivity and re-zero
if necessary. If the detector cannot be set to zero, use the
coarse zero control.
4. Standardize the bridge if necessary (See D - Stsdardiza-
z, following).
5. Insert the component to be measured in its test fixture.
Set the function switch to OPRRATE when using the self-contained
shielded measuring compartment, or set it to EWERNALOPFJWCE
when the unknown is located in the Model 51.52 Ren&e Test Chamber
or in another external sample holder. See E. Connecting to the
Unknown Resistor, which follows for detailed instructions for
connecting the unknown.
6. Set the BRnaE MU16 to the desired value. For external
bridge supplies, see F. External. Bridge Voltage Supply following.
7. Operate the FWH M RENI button and b&Lance for null with
the resistsnce dials. Increase the null detector sensitivity
(See H.Aa?uracy) to give the desired accuracy at final belsxxe.
The resistance of the comnent is then read. directly from the
resistence dlaJ.8.
Use x M to x O.OOldi
515 III - 3
D. STAEDARDIZ4TION
Wire-wound resistors have the greatest accuracy and keep their cali-
brations over long periods of time. Values greater then about one-
megohm, however, are too large and too expens e to be widely used.
Carbon film resistors provide values up to 10ei ohms and higher with
reasonable SUCC~QS this type resistor is used in the Model 515.
and
But then value of these resistors changes wlth time, sometimes one
or two percent per year.
The Keithley Model 515 MegohmBridge has been designed so that fre-
quent compensations can be made for variations of its high-megohm
standard resistors. This process is called Standardization and is
carried out as given below. Section IV - Circuit Description dis-
cusses the circuitry involved.
The bridge should be restandardized following a chenge in tempera-
ture of greater than about lOoF, and at least once each week, to
compensate for the errors introduced in the carbon standards by tem-
perature and time. For the utmost accuracy possible from the bridge,
it can be standardized. daily, hourly, or immediately before a crit-
ical measurement.
To Standardize the Bridge:
1. Set the Multi lier (the farthest right of the RESISTANCE,
OfJim dials) to 108 .
2. Set TXINCTION
switch to STANDARDIZE
3. Set NCLtLDETECTOR 1000 mv per division.
to
4. Operate IUSH ltl READswitch and balance the bridge as in
normal operation. The reading will be close to 10.00. The
final balance should be made with maxinnunnull sensitivity.
5. Release BUSHTo READswitch and set FUNCTIONswitch to
CWBRATE.
6., Operate PUSHTo READswitch and re-balance the bridge with
the 10 CALIBRQE potentiometer located in the Shielded Measur-
ing Compartment.
7. Turn FUNCTIONswitch back to STANDARDIZE.
8. Turn multiplier to I.07 and repeat steps 4 thru 7. Do this
for each successive mult lier thru 1011. The 1Ou position
is ot used since the 10f?i ohm standard is calibrated in the
lo I.? multiplier position.
E. COIVNKTINGTREUEKWOWWR.E3IS'lrOR
1.~ Using Internal Test Chsmber
515 III - 4
Fig. 1 Shielded Measuring Compartment, With Unknown Installed
The bottom section of the bridge contains the shielded compart-
ment for holding the unknown resistor, and is accessible when
the hinged door has been opened. The compartment has been de-
signed for greatest user convenience. Its being shielded elim-
bates troublesome pickup, and the unit construction eliminates
the necessity for having cables running from the unknown to
the bridge, with their associated flexure noise.
The measuring compartment will accept resistors up to about
eight inches long. Connections to the bridge are made through
banana jacks. A convenient clip to use with the banana jack
is the readily available Grayhlll Test Clip #2,1; it has a ban-
sna plug on the bottom snd spring clips on the top for holdin@;
the resistor heads; three are supplied with each bridge. These
clips are illustrated in Fig. 1, holding a typical high-megohm
resistor.
A number of ground jacks have been provided so that the ground
clip can readily be placed for conveniently holding the unknown
resistor, irrespective of its length.
In measuring high resistances, the many precautions necessary
in electrometer techniques must be borne in mind; most important
are the need for dryness and cleanliness so that leakage resist-
ance paths from the HI terminal to ground will not affect the
accuracy of measurement, and mounting the resistor so that its
body does not touch conductors or other insulators setting up
undesired or inadvertent leakage paths.
53.5 III - 5
2. Unknown Resistance Ecternal to ths Bridge.
The Moe1 51.52 Remote Test Chamber shown in Fig. 2 Is used for
testing irmilation or making other external shielded measure-
ments.: This test chamber is equipped with an integral &I-Inch
teflontinsulated trisxial cable fitted with a conuector for
attaching to the mating cqnnector in the Shielded Measuring
Compar&nentlnthebr.Mge. The chamber and connecting cable
are rated for continous operation at temperatures as Wgh as
125w.
Flg. 2.Model 5152 Remote Test Chamber
The eLectrical connections are made throngh banana jacks in
the oh&her. The hw.yhilJ. #2-1Test Clips as shown in Fig. 2
are furnished to facilttwte installing unkuowns tith axisJ~lea&s.
The banana jacks of course, can be used with any other connectors
or resistor holders.
To use:the Model 5152 Remote Test Chamber, fasten its cable
connector into the mating connector located in the Measuring
Compartment in the Bridge, and connect the unknown resistor
betweeq the RI and GROUND banana jacks in the Test Chamber (us-
ing the Cks+yhil.l test clips if possibae).
The third bsnsz.%jack in the Renwbe Test Chamber Is QUARD;it
Is cdn@ected through the inner shield braid of ths triaxial
cable to the guard connection in the Bridge.
Guarding is used exbenslvely in the Bridge to reduce the e&or6
causedfby spurions Le&age cwrents. WaxIing should also be
[email protected] in the construction of test electrodes fitted to the
RemoteITest Chamber, in order to obtain the greatest accuracy
from We bridge measurement,
515 III - 6
The guard conductors are driven from the galvanometer junction
of the low tiedance standard arms of the bridge; a total re-
sistance less than 10" ohms from guard to gmund till stit
the standards sufficiently to create errors great enough to
impair the rated accuracy of the bridge. Great care has been
taken in the construction of the bridge to kee the GUARD to
GROUND resistance substantial& higher than 10Yl ohms, and cexe
should be taken by the user to maintain that high level.
Fig. 3 is a simplified schematic diagram showin@;the electrical
connections of the standard and readout resistors, the unknown,
the null detector, and the guarding. A more extensive discus-
sion of the circuit operation and guarding will be found in
Section IV Circuit Description.
Fig. 3 Model 515 MegohmBridge, Simplified Schematic Diagram
In cases where measurements with the unknown external to the
bridge are necessary and the Model 5152 Remote Test Chambe?
is not suitable, the user can make his own holding fixture
and connect it to the bridge.
Teflon insulded trisxial cable should be used for the con-
nection. The central conductor is the High Impedance conductor;
the inner shield braid is the Guard, and is driven from the
low impedance arms of the bridge; and the outer braid is GND,
to provide shielding. Amphenol 21-529 is a suitable cable.
515 III - 7
The connector should also be teflon insulated. Gremar 7991
is satisfactory. Fig. 4 shows the connector and cable.
F. MTERWALBRIIGEVOLTAGESUPPLY
Bridge voltages higher than the 10 volts available from the internal
4 are desired when measuring resistances greater than about
;;r; ohms, or in studying the voltage coefficient of a resistor.
A UliF connector labelled EXTERN& LLNFVT mounted on the rear of
is
the bridge cabinet for ready connection of a high voltage source.
The shell of the connector is at ground potential, and this grounds
one terminal of the external bridge supp4. The central conductor
is the high - voltage lead. The bridge is insulated so that the
external bridge voltage can be as high as 1000 volts.
Either the Keithley Model 241 or the 240 Regulated High Voltage Sup-
ply makes a very satisfactory source for external bridge voltage.
The over-current protection on each is an important feature in pre-
xenting damage to the bridge resistors or to the unknown.
Whenusing externalbridge supply, setBRII?GEVOLTSto EXTafter
connecting the supply to the UHF receptacle on the rear panel. Do
not app4 more than 100 volts unless the x ICC or the x 10 dial is
in a position other than "O", for too much current will flow through
the bridge resistors. With the recommendedModel 240 or 241 Regulated
Voltage Supp4 the over current protection will prevent damage in
the eventthis precaution is not observed.
In making voltage coefficient measurements, it should be kept in
mind that the voltage applied to a Wheatstone Bridge is greater than
the voltage appearing across the unknown resistance being measured.
The relationship between the bridge voltage and the voltage across
the unknown is given in Section 0, below.
The shielded measuringcompartment in the bridge has a safety switch
which is operated when the door is closed. This switch operates
a relay whiah applies the voltage from the external bridge supply
to the bridge circuit. With the door open, the voltage is renmved,
so that the unknown can be changed without possible harm to the operator.
When the unknown is located outside the bridge, and the FUNCTION
switch set to MTERNALOPERATE,this safety interlock is removed fmm
the circuit< Unless the external bridge voltage supply is turned
off or disconnected, voltages dangerous to the operator may be present
at the unknown terminals. A convenient means of disconnecting the
source is to switch the BRIDGEVOLTAGE from EXT to zero.
G. MLTAGEAC!RGSSURKD3WNARDSTANBARB
In many cases, particular4 in measuring coefficients of resistors.
it is important to know the voltege across the unknown. In measuring
515 III - 8
the leakage resistance of capacitors, the applied voltage must be
known to avoid breakdown. Also, for rated accuracy, the voltage
across the standard resistor must not exceed ten volts.
If the bridge voltage is E, the unknown resistance X, and the stand-
ard resistance S, then the voltage across the unknown is:
and the voltage across the standard is:
The bridge voltage is read from the BRImE VOLTSdial or from the
external bridge voltage supply. The standard resistance is the value
indicated by the m&tiplier dial.
H. ACCUHACYCONSIDEHATIONB
The accuracy of measurement of an unknown resistor in a Wheatstone
Bridge depends primarily on the accuracy and stability of the other
three arms in the bridge, upon the resolution of the variable arm,
and upon the ability of the null detector to respond to the small
incremental changes in the variable arm. There are also numerous
secondary effects. These will aU be discussed below.
1. Null Detector Sensitivity.
To be able to detect a desired fractional deviation of the un-
known, corresponding to the wanted percent accuracy of the measure-
ment, the required null detector sensitivity is given by the
approximate expression*:
e is the null detector signal in volts
x is the incremental pa-t of the unknown resistance
E is the Bridge Potential in volts
S is the Standard Resistance, in ohms
X is the Unknown Resistance, in ohms
For resolutions of O.l$ in the unknown,
x = 0.00l.x
If X and S are approximately equal, and the Bridge Potential
is 10 volts,
o.oolx = 0.0015,
e = 0.0025 v0its
(2.5 millivolts)
*See Electrical Measurements by F. K. Harris. John Wiley & Sons, N. Y. 19.52
OY63R 515 III - y
In the case when X is approximately 10 S,
e = .W8 volts
(o .8 miuvolts)
The maximum sensitivity of the null detector in the Model 515
MegohmBridge is one millivolt per meter dial division, and
is thus sufficient for the rated accuracy of the bridge. Care
should be taken, however, to be certain that the detector sen-
sitivity and the bridge potential are great enough end the re-
sistance of X end S are sufficiently close to each other to
obtain the expected accuracy of measurement.
A check on the sensitivity of the system may be made by unbalanc-
ing the bridge readout dials a given percentage and observing
the null detector deflection.
2. Null Detector Zero Drift.
Vacuum tube electrometers drift about one to two millivolts
per hour, and this rate can be expected in the Null Detector.
Obviously, a false balance is indicated if the meter points
to zero, indicating balance, when in reality thereare several
millivolts at the input.
This error is easily eliminated by adjusting the null detector
to zero,whlle the PUSH7.0 READbutton is released, then depres-
sing the button end balancing the bridge.
3. Resolution of the Readout.
Using only the readout dials x10 through x.001, full rotation
of the x.001 dial is O.l$ of the total setting. The dial can
be easily read to one-twentieth of its full rotation, giving
a readout resolution of 0.005%. This is ten times the best
accuracy specified for any range.
Whenusing all the dials, the readout resolution is very much
greater than the maximumaccuracy.
4. Accuracy of the Readout Resistors,the Standard Resistors,
end the Standard Calibration Controls. (See Fig. 3 for the
location of each of them in the Wheatstone Bridge Circuit).
The accuracy of the resistors on the switches controlled by
each Readout Dial (RFSISTANCE,OHMS)is:
515 III - 10
Xl00 x.01 X.001
0.5% O%$ 0.1% 1.0%
ma~i.smmaccuracy with the bridge is obtained when using the
dials xlOthroughx.001. !l!his is because the most accurate
readout resistors are used, and also because the unknown re-
sistance and the standard resistance are sufficiently close
that the null detector has enough sensitivity with bridge Wit-
eges less than ten volts (see Section III R. 1).
!Chexl00 dial has only 0.5% resistors associated with it
because of the extremely high cost of more act te high
Y
value resistors, and because resistors above 10 ohms
are not very stable high accuracy measurements are not
warranted.
The resistors on the x.01 end x.001 dials are less accurate
because they are not followed by enough diels to give high
resolution, and their accuracies are great enough for rated
accuracy when using the x1.0 to x.001 dials.
With the MCLTIPLIER dial in either the 105 or 106 position
all three aTm6 in the bridge itself are wirewound resistors
accurate to .02f&, permitting the unknown to be measured to an
accuracy of .05$.
With the MUTEIPL.lRR the lo7 position, after the Standardiza-
in
tion process, the bridge accuracy is that of the previous re,nge
(.05$) plus the error introduced by standardizing, which is
conservatively set at .05$.
FoUowing this pattern, the accuracy of the bridge at each suc-
cessive step of the lrmltiplier dial is the accuracy of the pre-
vious step plus the .05$ standardizing error. It is in this
fashion that the accuracies in the specifications up thru 1012
ohms were derived.
FromlO~tto l.014 ohms, enough secondaryl~ffects are present
to warrant the 0.5% rating, and above I.0 ohms, the xl00 dial
is used, adding enough further error to bring the overall accuracy
rating to l.O$.
The standard resistors used are as follows:
Multiplier Resistor !Pype Accuracy
Wire Wound
0 (1 0.0s
q 0.0s
Dezosit,ed Carbon 1.0 %
3 1.0 5
Sealed,?-Meg
II 2.0 $3
30 2.0 $
I& I! II
ld2 II 11
515 III - XL
The Standard Calibration Controls arm is either a wirewound
resistor accurate to 0.02$, or deposited carbon resistors in
series with trinnaing potentiometers.
5. Temperature and Voltage Coefficient of Bridge Resistors.
The wirewqind resistors employed are free from voltage coefficient.
They use one of the lowest temperature coefficient of resistance
alloys available, changing 20 parts per million per 'C, or O.OG~$,/~C.
They are measured at room temperature, 25oc, end for greatest
accuracy, the bridge should be used near this temperature.
The depositedcarbon and Hi-Meg resistors have substantially
higher temperatuqe coefficients of resistance than the wire-
wound resistors. But if the bridge is allowed to come to its
working temperature and standardized, it will have its rated
accuracy unless the temperature changes. In this case it should
be restandardized.
Deposited carbon and Hi-Meg resistors also exhibit voltage co-
efficientof resistance. The'Hi-Meg resistors used in the bridge
are spiralled and have about one tenth the voltage coefficient
of standard Hi-Meg units. Nevertheless, the voltage across
these resistors should not exceed 10 volts for ms.xitmsnaccur-
acy . Seem Section G. Voltage Across Unknown and Standard Re-
sisters .
6. Leakage Resistence Across the Unknown.
lOlo ohms shunting one megohm(I.0 6 ohms) produces a change of
0.015; and 1015 ohms shunting 1012 ohms produces a 0.1% change.
With high resistance resistors end high accuracies leakage re-
sistance is an important consideration.
The termipals of the Model 515 MegohmBridge have been carefully
made with teflon insulation, and guarding has been employed
extensiveily . The major concern of the operator in using the
bridge is to keep the insulation clean and dry. The user, how-
ever, should be greatly concerned with the bobbin and housing
or casing of his unlrnown resistor and with any specielly built
holdllng fixture. Paper base bakelite which has been handled
and allowed to remain in a humid atmosphere has a surprisingly
low resistance. Glass envelopes which have been handled and
have finger oil and salt paths between fused-in wire conductors,
or simple water vapor paths, also csn have a surprisingly low
resistance. E&reme care is necessary to avoid unsuspected
errors or: instabilities in measuring high resistances.
7. Errors Caused by Guard to Ground Resistance.
Guarding, as described in Fig. 3 Section III E, is used exten-
sively in.the construction of the Model 515 MegohmBridge to
515 III - l2
reduce errors caused by undesired leakege currents. The Guard
conductors are driven from the low impedance side of the Null
Detector. Resistance from Guerd to Ground shunts the resistors
on the Readout Mel switches and the Standard Calibration con-
trols. The Readout Mal resistors of xl0 to x.OClmay be as
hlghas10me@ms
shunted with 5 x of ~~~nt~~~$o~~
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