ELECTROMOTIVE FORCES IN THE VOLTAIC CELL. 



473 



position very clearly, and records a series of determinations of Volta force 

 among metals, determinations which are evidently the most accurate and 

 satisfactory yet made. He adopts the capital experimental method of 



FIG. 6. -Diagram of Pellat's Method. 



m interrupteur. 

 i' feuille d'or. 



LEGENDE. 



kl compensateur. 



curseur. 



R rheostat. _ A et a' plateaux attractifs de 1'olcctroscope. 



p piles Darnell fourmssant le eourant du l b b batteries de 100 volts chacune pour 



compensateur. charger les plateaux a a'. 



p p plateaux du condensateur (p fixe, p' I 



mobile). I 



The diagram Fig. 6 pretty well explains itself. The contact at m is broken the instant before- 

 p' is raised. 



Fig. 7. — Pellat's Condenser. 



Fig. 7 shows how this was done in practice. Pulling the string a raises a sliding pin with a 

 shoulder q fixed to it a millimetre below the bar B. The pin thus tirst breaks'contact near 

 H, and then raises the hinged bar p. b, to which the upper plate m is attached. The upper 

 plate, being always in connection with a battery, requires no special insulation. The lower 

 plate is carefully supported by a Thomson insulator i. The lower plate was considerably 

 smaller than the upper one, and it was further protected from stray inductions bv a guard 

 screen. Screws c c served to regulate the distance between the plates. 



neutralising the charge of a condenser by a Poggendorff or compensation 

 method, and thus converts Kohlrausch's into a null method, for which a 

 very sensitive electroscope is all that is needed. The plates of the two 



Louis le Grand, No. 461 : Juin 22, 1881. See also Journal de Physique, 1881, xvi. p. 68, 

 and May 1880. ' Difference de potentiel des couches electriques qui recouvrent deux 

 metaux en contact.' 



