MEASUREMENT OF RESISTANCES. 



265 



Sat J at 500, hold the telephone to your ear (two telephones are 

 better), and adjust R until the sound fades to the lowest the resist- 

 ance will permit, e.g., find the two resistances between which there is 

 silence ; then move J until two points are again found between which 

 silence occurs. 



Then, as the lengths of wire on each side of J are to each other, 

 so is R to X. 



The inductorium must be placed in an adjacent room so that it be 

 not directly audible, and the S coil must be adjusted to give a well 

 marked sound in the telephone. If two telephones are employed, one 

 to each ear, a collaborator will be required to adjust the resistances. 



Measurement of resistance of a g-alvanometer or cell by the 

 half deflection method. 



GALVANOMETER BATTERY 



FIG. 61. 



(a) Galvanometer: Battery (1 Daniell) short-circuited on closure of 

 key through the shunt wire S. R variable resistance. G the galvano- 

 meter. Vary S (No. 18 wire, 25 to 30 cm long, with G about 6000 ohms) 

 until the deflection of the galvanometer is about 20 cm on the scale. 



Then increase R until the deflection is reduced to one-half. The added 

 resistance R will equal the resistance of the galvanometer. 



(b) Battery : Transfer the shunt wire S to the terminals of the galvano- 

 meter (No. 14 wire, a straight piece) and adjust to the same deflection as 

 before. Increase R until half the deflection is obtained. R will represent 

 the resistance of the battery. 



It is best to take an odd number of batteries, 5 or 7 coupled end 

 on, with an even number coupled in opposition zinc to zinc, so that 

 there will be E.M.F. only from one cell in the G circuit. 



The total resistance divided by the number of cells gives the average 

 resistance for one. 



