176 a:nnual kecord of science and industry. 



hanging in the room, the thickness of the wire being suffi- 

 ciently great to prevent any rise of temperature in it. By 

 means of a Thomson quadrant electrometer the difference 

 of potential between the carbon points was measured, and 

 compared with that between the wire of known resistance. 

 Since at any given moment the same current is flowing 

 through the electric arc and the wire, the differences of po- 

 tentials are proportional to the resistances. The alternate 

 measurements succeeded each other closely, and were repeat- 

 ed many times. The resistance obtained never exceeded 

 twenty ohms, and had an average of about twelve ohms ; 

 the variation being considerable even when the light was 

 apparently steady. In a second series of experiments, eighty 

 Grove cells joined in series, which had been in use occasion- 

 ally for three hours after being set up, were connected with 

 a differential galvanometer, in one circuit of which was a 

 high resistance, in the other the electric arc, each galva- 

 nometer-coil beins: shunted with a wire of small resistance. 

 When balance was obtained, the resistance of the arc, which, 

 as before, varied greatly, never exceeded twenty-nine ohms, 

 and averaged twenty ohms. From these results the authors 

 draw the important conclusion that with such cells as they 

 used no attempt should be made to join any of the cells in 

 parallel circuit until at least two hundred have been joined in 

 series; for if the resistance of each cell is about 0.2 ohm, two 

 hundred of them would have a resistance of forty ohms a re- 

 sistance certainly less than double the electric arc for that bat- 

 tery corresponding with brightest light. Moreover, the au- 

 thors have previously shown that the cells of a battery should 

 be joined in series until the resistance of the battery is double 

 the external resistance, at which point the battery should be 

 joined in two rows, each containing half the number of cells 

 in series, and the two rows connected in parallel circuit. 12 

 A,October 19, 1876, 544. 



ACTIOX OF LIGHT ON EBONITE. 



Herbert McLeod has called attention to the production of 

 acid drops upon surfaces of ebonite or vulcanite exposed to 

 the air by the action of light. It has long been known that 

 the insulation of electrical apparatus made of this material is 

 impaired by the formation of this conducting layer, w^hich is 



