Van Name — A Rheostat for Large Currents. 349 



Art. XXIX. — A Rheostat of Variable Range, Suitable for 

 Large Currents ; by K. G-. Tan Name. 



[Contributions from the Kent Chemical Laboratory of Yale Univ. — cclxii.] 



In experiments involving the accurate regulation of currents 

 of one hundred amperes and above, it is often a matter of 

 some difficulty to provide a suitable rheostat. The commer- 

 cial types are expensive, and substitutes operating on the 

 metallic resistance and sliding contact principle are not easy to 

 make, on account of the necessity for very substantial construc- 

 tion of the contacts. Electrolytic resistances (iron plates in 

 sodium carbonate solution, and the like), though very easily 

 constructed, are not always satisfactory. When used with 

 direct current their resistance cannot, on account of the polari- 

 zation, be gradually reduced to zero as with the other types of 

 rheostat, and this is often a disadvantage, especially with low- 

 voltage currents, and in general when economy of energy is a 

 consideration. 



Below is described a form of rheostat which may be employed 

 under a wide range of conditions, but is especially suitable for 

 large currents, including those of low voltage for which an 

 electrolytic rheostat is nearly or wholly useless. It was 

 devised and used by the writer for the control of a 25-volt 

 current of about 200 amperes, in studying the electrolysis of 

 fused calcium chloride. This rheostat possesses the advantage 

 that the resistance element can be quickly removed and 

 another of different dimensions inserted, thus altering the 

 range. Extra resistors are easily made, at trifling cost, and 

 may be used indefinitely. 



Essentially, the rheostat consists of a curved rod of graphite, 

 submerged in water and so mounted that its effective length is 

 varied by rotating an axle which serves at the same time as 

 one of the current leads. The graphite resistor, which has the 

 general form shown in figure 1, is sawed out of a plate of 

 Aeheson graphite, conveniently J inch thick. It is centrally 

 mounted on a stout horizontal axle, so that its rim runs with 

 liberal clearance in a suitably shaped hollow milled in a block 

 of copper. This hollow is filled with mercury, thus providing 

 electric contact between the copper and graphite. Cooling is 

 effected by enclosing the whole in a wooden box through 

 which water circulates. 



It is evident that by rotating the axle the resistance inter- 

 posed between the axle and the copper block may be varied 

 from practically zero up to a maximum determined by the 

 shape and dimensions of the graphite resistor, in particular by 

 the thinness of its " tail." Since this part of the resistor is 

 not subject to any mechanical strain, it may, if desired, be 



