[HENRY] CONTACT RESISTANCE BETWEEN CONDUCTORS 137 
A scale and a reflected spot of light were used to indicate the 
current passing through the galvanometer. 
K, is a reversing key. 
K2 is a key to break the circuit. 
R is the relay consisting of D the revolving disc, N the needle 
(used in making contact), C the moving coil of the relay, S the shaft 
through the disc to the spring, T the terminals of the main circuit, 
T; the terminals of the relay coil. 
R, is a standard resistance containing coils from 1 to 100 ohms. 
R: is a standard resistance box in the form of a Wheatstone’s 
Bridge. 
APPLICATION OF PRESSURE. 
To apply various pressures between the needle and the rotating 
disc electric currents of various strengths were passed through the 
moving coil of the relay. The pressure with which the needle was 
held against the disc was proportional to the current passing through 
the coil of the relay, since the deflection of the coil and hence the de- 
flection of the needle depended on the current flowing through the 
coil. This deflecting current was obtained from a circuit composed 
of the storage cell B: (Fig. 1), a standard resistance box Ry, with 
coils ranging from 1-100 ohms in series with a resistance box R2 in 
the form of a Wheatstone’s Bridge used as a potentiometer. Hence 
being able to change the resistance in the potentiometer and in the 
100 ohm resistance box it was possible to keep the total current in the 
circuit constant and yet vary the current through the relay coil so 
as to obtain various pressures. 
The method of measuring the contact resistance was that of 
“substitution.” A standard resistance box was substituted for the 
contact wires, and by putting into the circuit various coils of the re- 
sistance box it was possible to calibrate the scale for all deflections, 
corresponding to resistances from infinity to zero. The megohm 
prevented any sudden change of the E.M.F. of the battery, so that 
there was little or no change even after the battery had been in use 
for several days. The resistance of the galvanometer coils was 
about 84 ohms. The deflection when the contact wires were in the 
circuit was noted and from the curve the resistance was obtained. 
ROTATION OF CONTACT. 
The disc was rotated by means of a shaft attached to a very heavy 
steel motor clock. It was possible to obtain uniform speed of rotation 
by keeping the spring tightly wound. The spring when once wound 
tight could apply a driving force for a period of twenty-four hours. 
