Measurement of charge of artificial torpedo 201 



ment by electrifying the second jar, communicating its electricity to the 

 first, and observing how far the balls separated; the mean between these 

 two distances will evidently be the degree of separation sought, though 

 the jars were not of the same size. Having found this, I electrified one row 

 of the battery till the balls separated to the first distance, and repeatedly 

 communicated its electricity to the plate of coated crown glass, taking 

 care to discharge the plate each time before the communication was made, 

 till it appeared by the electrometer, that the quantity of electricity in 

 that row was reduced to one-half. I found it necessary to do this between 

 ii or 12 times, or n times as I estimate it. Whence the quantity of 

 electric fluid in the row may be thus determined. 



413] Let the quantity in the plate be to that in the row as x to i ; 

 it is plain, that the electricity in the row will be diminished each time it 

 is communicated to the plate, in the proportion of i to i + x, and conse- 

 quently after being communicated iij times will be reduced in the 



_ _i 



proportion of i to (i + #)"*; therefore, (i + x)"* = 2; and i + x = 2)"*. 

 Whence the value of x may easily be found by logarithms. But the readiest 

 way of computing it, and which is exact enough for the purpose, is this: 

 multiply the number of times which you communicated the electricity of 

 the row to the plate, by 1,444 '> an d from the product subtract the fraction ; 



the remainder is equal to - , or the number of times by which the elec- 



X 



tricity in the row exceeds that in the plate*. 



414] The way by which I estimated the strength of the charge given 

 to the battery, was taking a certain number of jars, and electrifying them 

 till the balls of the electrometer separated to a given distance, and then 

 communicating their electricity to the battery. This method proved very 

 convenient; for by using always the same jars, I was sure to give always 

 the same charge with great exactness; and by varying the number and 

 size of the jars, I could vary the charge at pleasure, and besides could 

 estimate pretty nearly the proportion of the different charges to each 

 other. It was also the only convenient method which occurred to me ; for 

 I could not have done it conveniently by charging the whole battery till 

 an electrometer suspended from it separated to a given distance ; because 

 in most of the experiments the electricity was so weak, that a pair of fine 

 pith balls suspended from the battery would separate only to a very small 

 distance ; and counting the number of revolutions of the electrical machine 

 is a very fallacious method. 



415] I found, upon trial f, that though a shock might be procured 

 from this artificial torpedo, while held under water, yet there was too 

 great a disproportion between its strength, when received this way, and 



* [Arts. 441, 582.] t [Art. 596.] 



