350 Van Name — A Rheostat for' Large Currents. 



given a very small cross section. The axle may conveniently 

 consist of a small steel circular saw arbor, which can be pur- 

 chased ready-made for two or three dollars, and comes fitted 

 with a suitable device for holding the resistor. It is desirable, 

 though not essential, to electroplate the arbor at the outset as 

 a protection against rusting. 



Further details of construction are given by the following 

 description and figure of the rheostat used by the writer in 

 the work above mentioned. Figure 2 is a side view of the 



Fig. 1. 



ij: 



v-ts-^. ----------- -- 



apparatus, and except for a small part drawn in section, shows 

 it as it would appear if one end of the enclosing box were 

 removed. The resistor A was of Acheson graphite, 6 inches 

 in diameter and \ inch thick; the steel axle B a {-inch circular 

 saw arbor, threaded at one end to receive the copper terminal 

 lug O, but otherwise used just as purchased. Washers of 

 sheet copper D D were inserted between the arbor and the 

 resistor to improve the electrical contact. The axle was sup- 

 ported by the bearing E, a stout brass casting, and kept from 

 longitudinal motion by the two brass collars FF, held by set 

 screws. The bearing E was mounted on the wooden base 

 block 6r, as was also the copper block H which contained the 

 mercury contact, the copper block imbedded in the wood 

 slightly below its surface, and connected by copper bolts «/, 

 passing through the wooden base, to the copper strip iTbelow, 

 which served as one of the current terminals.* Wooden sides 

 L Z, and end pieces, attached to the base block G, formed a 

 box about 12 by 5 inches and 6 inches deep, inside measur- 

 ment. When the rheostat was in use a stream of water, 

 sufficiently rapid to keep the box full and to prevent excessive 

 heating of the resistor, was admitted by a tube inserted into 

 the box from above, and passed off by an overflow. The box 

 did not need to be watertight, since the apparatus, mounted on 



* Direct currents may be connected to flow through the rheostat in either 

 direction, but it is better, when practicable, to connect the axle with the 

 negative pole, as a protection against corrosion by the working current. 



