64 
PROFESSOR HUGH L. CALLENDAR ON 
terminals A and B, to 100,000 ohms. The ends of each of the 20-ohm coils of the 
vernier dial are connected to platinized studs arranged in a circle, which make 
contact one at a time with a single revolving contact spring, connected to the 
galvanometer terminal G. This arrangement of main and vernier dials permits the 
sub-division of each hundredth part of the whole resistance into one hundred parts, 
so that the reading of the two dials gives the P.D. to be measured directly to one 
part in ten thousand of the P. D. on the terminals AB. 
The advantages of this form of potentiometer, in addition to its high resistance, 
were (l) the great facility and rapidity of reading and manipulation, and (2) the 
symmetry of construction, which permitted a very high order of accuracy of calibration 
to be attained, and greatly facilitated the application of corrections, as compared 
with the usual type of instrument in which a bridge-wire is employed for the finer 
sub-divisions. 
In the use of this instrument in our experiments the P.D. to be measured seldom 
exceeded 4 volts. The terminals AB were permanently connected to three Leclanche 
cells, which gave a very steady current through so high a resistance. The reading of 
the two Clark cells employed as a standard varied by a few parts in 10,000 only from 
week to week, and generally remained constant to 1 in 100,000 for the short interval 
of 15 minutes corresponding to any single experiment. 
The galvanometer employed with this potentiometer had a resistance of 
110,000 ohms. The astaticism of the needles was adjusted as carefully as possible, so 
that the effect of disturbance of earth-currents due to the electric railway might be 
negligible. The suspended system was fitted with a very perfect mirror and a 
damper to make it practically dead-beat. The sensitiveness was adjusted by control 
magnets to give a deflection of approximately 10 scale-divisions for one division of 
the vernier dial (1 in 10,000). The perfection and steadiness of the image was such 
as to permit reading to a small fraction of a scale-division. The first four figures of 
the reading were given by the setting of the dial contacts. It w r as easy to estimate 
the fifth figure at any moment by inspection of the galvanometer deflection. The 
temperature conditions w T ere generally so steady in the course of an experiment, and 
the diminution of the electric current and the water-flow so gradual and regular, that 
it was possible, as a rule, to predict the reading of the P.D., either on the standard 
resistance or on the heating conductor, to 1 in 100,000 for at least five minutes 
ahead. 
As there were no observational difficulties to contend with in the electrical 
readings, the relative order of accuracy of the results would be limited only by the 
constancy of the Clark cells and the current standards, and by the order of accuracy 
attainable in the calibration of the potentiometer and in the permanence of the 
relative values of the coils. The coils of the main dial, which were the most 
important, were all precisely similar, wound with the same wire and carefully 
protected from sudden or unequal changes of temperature. The ratio of the 
