and Retractions of Rods of Iron and Steel, 189 



or without the intervention of a mercury cup, we at once could 

 suddenly send the current with its maximum intensity through 

 the helix. 



Mean results of first series of experiments. One cell in circuit. 

 Resistance of fine wire =*6 ohm. 



On gradually diminishing the resistance, 



Fraction of length of fine ) , a I 1 * n 



interposed wire . . J 4 " 2 * ** ¥ * 



Scale-readings . /"went from 54-8^ T 54 . 9 55 g5 . 3 55 . 6 M .j 



On gradually increasing the resistance, 



Scale-readings / went ^ ^t 1 55 ' 6 55 * 8 55 '9 55 ^5 561 



° L to 54*8 J 



Tangent-galvanometer . 4f ° . . 29^° 



Mean results of second series of experiments. One cell in cir- 

 cuit. Resistance of fine wire = # 9 ohm. 



On gradually diminishing the resistance, 

 Scale-readings .... 54'8 54-8 54'85 55 55'4 56'1 



On gradually increasing the resistance, 



Scale-readings | wentfro ^^g 5 l 55-4 55'65 55-8 55*9 56*1 

 Tangent-galvanometer . . 3° 29J° 



Examining the results in the two series bf experiments, we 

 see that when the current was passed with all of the interposed 

 resistance in the circuit, the scale went from 54*8 to 54' 85, or 

 moved *05 of a division in the first series of experiments; but 

 in the second series the current was too feeble to effect a mea- 

 surable elongation, and it was not until one fourth of the fine 

 wire was out of the circuit that the scale-readings began to in- 

 crease. In both series of experiments the rapid increase in the 

 rate of elongation is noticeable after three fourths of the fine 

 wire was out of the circuit. After all the interposed resistance 

 had been traversed and was out of the circuit, the elongation in 

 both series of experiments amounted to 1*3 division of the scale. 

 The same amount of elongation always occurred when the ends 

 of the copper wires were brought together, or when the circuit 

 was as suddenly formed by plunging the wires into a cup con- 

 taining mercury. Therefore it is well established that a current 

 of a definite strength will produce the same amount of elonga- 

 tion, whether that strength is suddenly or gradually attained. 

 Indeed in some of the experiments over three minutes were oc- 



