1840] WRITINGS OF JOSEPH HENRY. 177 



short metallic conductor, it may be neglected, and then the 



expression becomes 



nA A 



or --; 



rn r 



and since this expresses the quantity of current electricity in 

 a unit of the length of the circuit, with either a single or a 

 compound battery, therefore with a short conductor the quan- 

 tity of current electricity in the two cases is nearly the same. 



72. Let us next return to the experiment with a battery 

 of a single element, (68,) and instead of increasing the in- 

 tensity of the apparatus, as in the last example, let the 

 length of the conductor be increased ; then the intensity of 

 the shock at the beginning of the current, as we have seen, 

 (14,) will be diminished, while that of the one at the ending 

 will be increased. That the shock should be lessened at the 

 beginning, by increasing the length of the conductor, is not 

 surprising, since as we might suppose, the increased resist- 

 ance to conduction would diminish the rapidity of the de- 

 velopment of the current. But the secondary current, which 

 is produced in the conductor of the primary current itself, 

 as we have seen, (19,) is the principal cause which lessens the 

 intensity of the shock ; and the effect of this, as will be 

 shown hereafter, may also be inferred from the j^rinciples 

 we have adopted. 



73. The explanation of the increased shock at the moment 

 of breaking the circuit with the long conductor, rests on the 

 assumption before mentioned, (69,) that the velocity of the 

 diminution of a current is nearly the same in the case of 

 a long conductor as in that of a short one. But to under- 

 stand the application of this principle more minutely, we 

 must refer to the change which takes place in the quantity 

 of the current in the conductor by varying its length ; and 

 this will be given by another application of the formula be- 

 fore stated, (71.) This, in the case of a single battery, in 

 which n equals unity, becomes 



A 

 r + R' 



and since this, as will be recollected, represents the quantity 

 12 



