Exhibit 398 is a rheostat, a resistance variable in 8 steps selectable by a metal arm with a handle. The resistances are 8 double spirals of constantan (45% Nickel and 55% copper, a desirable choice since the resistance varies very little with temperature). There are three terminals, two on the extreme ends and one attached to the contact arm.r
BIBLIOGRAPHY
[1] E. Perucca, Fisica Generale e Sperimentale, vol. II, UTET, Torino, 1937, pag. 594.
[2] L. Graetz - L'elettricità e le sue applicazioni - Vallardi Editore, Milano - 1909.
