RESISTANCE BY A METHOD BASED ON THAT OF LORENZ. 
95 
This was tested by completing the main circuit through the leads only (omitting the 
coils) and taking a few complete sets of observations. No effect could be measured 
and we conclude that the current in the concentric leads and remaining portions of the 
circuit other than the coils have no influence on our results. 
(2) Normal Procedure in making cm Absolute Measurement of a Resistance. —In 
making a determination of a resistance in absolute measure, we determined (a) the 
insulation resistance of the various parts of the circuits ; ( b ) the mean distance between 
the mean diametral planes of the coils ; (c) the temperature of the marble cylinders ; 
( d ) the want of equality between the product of speed and mutual inductance and the 
resistance, the value of which was desired ; ( e ) the temperature of the marble 
cylinder; and (f) repetition of (6). 
To measure (b) and (f) we commenced with observations on the invar line standard ; 
we then observed the eight plug distances and concluded with further readings on the 
invar. These observations lasted about twenty minutes. 
The temperatures (c) and (e) of the cylinders were taken by means of thermometers, 
and the effective temperature was calculated by means of the formula given in 
Section 13. In general, the difference of temperature (e) —(c) was about 2°'2 C. To 
determine (d) the commutators and plugs were correctly set, the galvanometer circuit 
closed, and the reversals of current, &c., made which are indicated in Section 20. At 
the same time the speed was maintained constant within a few parts in 100,000 and a 
record of the speed taken. The time occupied was usually from 17 to 20 minutes. In 
general, the first 12 reversals of current produced changes in the deflection due to a 
difference Mu. —R where R is the value of the standard resistance in absolute measure. 
The next 8 reversals were made when R was reduced in value by 1 part in 1000 by 
shunting it with another standard resistance. Then followed 24 more readings with 
R unshunted, 8 with R shunted, and the final 12 measurements were made with R 
again normal. The sensitiveness of the arrangement is, of course, directly proportional 
to the current through the coils, and as this was not constant from day to day we 
thought it best to determine it on every occasion. 
Before proceeding with a measurement, the brushes were wiped with washdeather 
and the tension on the wires adjusted. The variations in the thermo-electric effects 
were then observed and the petrol lubrication attended to. Bad lubrication on disc 
No. 1 produced a deflection of the galvanometer in one direction, and bad lubrication 
on disc No. 2 produced a deflection in the reverse direction. With a good supply ol 
petrol on the edges of the discs there was practically no deflection when the brushes 
were in good condition. When the brushes were somewhat worn—usually after 
6 runs—it was not possible to take good observations, and rather than waste time in 
making indifferent measurements we preferred to wait until new brushes had been 
inserted. Careful tests of the brushes were regularly made, and it is no doubt due to 
this fact that we are able to record the result of every completed measurement. At 
times the thermo-electric and speed variation effects were a little troublesome, but we 
