June 19, 1879] 



NATURE 



177 



The circuit was first broken by removing the earth wire 

 from ring 17, and the plug T, in connection with the in- 

 duction plate I, being touched at any point between k' 

 and ring I gave the reading for " Full Potential, open 

 circuit ;" next, the earth wire was replaced at ring 17, 

 and the value for "Full Potential, closed circuit," was 



obtained by causing T to touch at K'. As these batteries 

 had but small internal resistance, the difference between 

 these two readings was scarcely perceptible. By touching 

 T at rings i, 2, &c., in succession, their potentials were 

 observed. The current was then reversed and similar 

 observations were made. Next, for the purpose of 



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 Fig. 5. 



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making a better examination of the tube in detail, the 

 induction plate I was lowered to that distance which gave 

 as large a deflection for the difference of potential between 

 the two ends of the tube as was convenient. After the 

 potentials of the several rings had been measured in suc- 

 cession with both currents, the induction plate was 



restored to its original position, and one or two of the 

 first observations were repeated for confirmation. The 

 current was then varied by altering the resistance in 

 circuit,' and fresh measurements made in the same order. 

 Thus the following values were obtained : — 



I. Circuit : — 2,400 rod-cells, i megohm resistance. Tube 150. Induction plate at 2 inches distance from the quadrant 



Zero 



Full potential (open circuit) 

 (closed ,, ) 



Pote: 



tial at ring I 

 .. 2 



3 

 „ 4 

 5 



14 

 IS 

 16 



17 



Current + 



6 right = o 



154 left =160, 



i53or4„ = 159: 



106 „ = 112: 



96 ,, = 102 



not observed 



47 



26 left - 



20 „ = 



16 „ = 



6 right = 



Current 

 6 right = 

 200 

 198 



133 

 108 

 100 



95 

 89 



— IQ2> 



32 



26 



22 



Oj 



O 

 194 

 192. 

 I27J 

 102 



94 

 89 



83 

 not observed 

 25 right — 19 

 20 „ = 14 

 6 „ = o; 



Differences. 



65 



127 



II. Circuit varied by substituting 800,000 ohms for the 1,000,000 ohms of wire, and inserting liquid resistance No. 3 



(2,690,000 ohms) between the wire resistance and ring i. 

 Current + 

 ist Observation. 2nd Observation. 



5 right =0 5 right 



159 left = 164 159 left 



l59or8„ =163 



139 .. ^ 144 



112 right = 117 



not observed 



158 

 138 



III „ 



103 ,, 



97 ,. 



90 .. 



25 ., 



18 „ 



5 right 



= o 



= 164) ... 



= 163/ 



= 143': ■■■ 



- 116; •■■ 

 = 108 



= 102 



= 95; - 



= 30 



= 23 



- 0} 



Mean Differences. 



195 



27 



= 187] 



2nd Observation. 



S right = o 

 192 

 191 

 167 

 136 



5 



I 



186, 

 162} 



•3» 



o 



116-5 



Mean Differences. 



I 



... 25-5 



... J32 



This method of varying the current is arranged to save time. The circuit must not be interrupted in the course of a set of observations. 



