258 PEOCEEDINGS OF THE AMERICAN ACADEMY. 



two distilled water resistances which enable one to control the strength 

 of current of the battery. The cells are charged in multiple with only 

 twenty in each branch of the divided circuit; and they are always left 

 on the multiple circuit when not in use. The resistance of each cell is 

 about one fourth of an ohm, and the electromotive force about 2.1 volts. 

 A study of the voltage of each case by means of a voltmeter showed that 

 the voltage could be closely estimated by knowing the number of cells. 



The Plante rheostatic machine, by means of which one can rise from 

 the voltage of the battery (20,000) to 500,000 volts,* is a practical 

 modification of that described by its author. Instead of mica plates, the 

 mechanician of the Laboratory, Mr. Thompson, selected glass plates 8 X 10 

 inches, one tenth of an inch thick : these were coated with tinfoil to 

 within one inch of the edges of the plate. At first it seemed doubt- 

 ful whether glass of this thickness would stand the high voltages to which 

 it would be subjected. Preliminary experiments, however, showed that 

 this thickness of glass would stand a steady stress of twenty thousand volts, 

 — although it would break down under much less voltage arising from 

 alternating stresses. I have employed thirty of such glass condensers or 

 Franklin plates, which are charged in multiple by the battery of from five 

 thousand to ten thousand cells ; that is, all the coatings of one side of 

 the plates are connected with the — pole of the battery, and the coat- 

 ings of the other side with the -f pole of the battery. The change 

 from multiple to series is accomplished by a series of brushes which are 

 arranged on two vulcanite rods. These brushes are connected with the 

 coatings of the glass plates. A revolving drum provided with pins and 

 connecting wires is driven by an electric motor, and serves rapidly to 

 change the connection of the condensers from multiple to series. 



Starting from the point reached by De La Rue and Muller, about 

 15,000 volts, the Plante rheostatic machine shows that the length of 

 spark is closely proportional to the electromotive force. On plotting 

 lengths of sparks as ordiuates, and JVV as abscissas, H being number 

 of Franklin Plates, a straight line is obtained. 



The method I have employed for studying the energy conditions in 

 Crookes tubes, and in the production of electrical discharges in general, 

 may be termed the damping of an additional spark method, or the com- 

 parison of resistances by the estimation of the damping of electrical oscil- 

 lations-t The electrical circuit is provided with two spark gaps. One 



* Comptes Rendus, Tom. LXXXV. p. 794, Oct. 29, 1877. 



t Damping of Electric Oscillations, These Proceedings, Vol. XXVI. 



