Conductivity of some Dielectrics. 491 



charged or discharged from passing through Gi and altering 

 its zero. The use of the two keys K 2 and K 2 also enables 

 the zeros of the galvanometers to be frequently tested. This is 

 desirable as all three galvanometers are provided with variable 

 shunts,, not shown in the figure, to reduce their sensitivity 

 to the desired extent, and small thermal effects at the shunt 

 terminals sometimes cause a shift of zero of a few divisions in 

 G x . The galvanometers G A and G 2 are protected, as shown, 

 by small inductance coils and safety spark-gaps H^ and H 2 , 

 each a small fraction of a millimetre, and also by the large 

 shunting condensers, to prevent damage in the event of a 

 failure of the specimen, or a flash over at the revolving 

 contact-maker. 



The galvanometer Gi has a coil resistance of 3290 ohms at 

 16° C. and a damping (shunt) resistance 12,490 ohms, so that 

 its effective resistance is very close to 2600 ohms. The 

 sensitivity with this damping coil in use is about 2100 scale- 

 divisions per microampere. It was standardized throughout 

 the range used, and a curve (differing very little from a 

 right line) plotted, enabling the current for any deflexion to 

 be read off. Its sensitivity could be reduced, when neces- 

 sary, to any desired extent by the use of an external shunt. 

 G 2 has a resistance of 296 ohms and is naturally dead-beat, 

 its coil being wound in a silver tube. Its sensitivity is about 

 135 scale-divisions per microampere and was reduced, when 

 necessary, by a 200 ohm shunt. All three galvanometers 

 are of the suspended coil type. 



The revolving contact-maker is shown in section in n>. 2. 

 It consists of a small parallel plate condenser formed of two 

 metal disks A and B to whose edges are soldered hollow 

 metal rings, seen in section in the figure, the over-all dia- 

 meter' being 9'1 cm. They are mounted, as shown, on the 

 spindle of a single phase induction motor so that A is 

 earthed through the spindle, while B is held between two 

 ebonite collars 3'3 cm. in diameter, the distance between 

 the disks being 8 mm. The steel rod C, electrically con- 

 nected to B by a small spring, makes contact in the course of 

 its revolution with two steel spring brushes, one of which 

 runs in each of the grooves shown in the vulcanite. These 

 brushes are insulated and connected as shown in fig. 1. As 

 the motor is running light its speed is only slightly below 

 that of synchronism, and hence may be taken as constant, 

 since the frequency of the A.C. supply is almost constant. 

 The speed must be about 24 revolutions per second. A carbon 

 filament lamp a short distance below the contact-maker keeps 



2 L2 



