36 THE ADDRESSES, LECTURES, ETC., Ofi 



fairly, you would make the two beams of the balance about 

 equal, for if one arm of the balance were short, and the other 

 long, you would not weigh at this short end with the same 

 degree of nicety. In the case of electricity another evil arises, 

 that the current, dividing itself unequally between the long arm 

 and the short arm, would heat the short arm ; therefore, we are 

 limited in the use of Wheatstone's diagram to a certain range 

 of resistance. We can produce, artificially, the resistance of a 

 thousand units ; and, therefore, can compare directly as though 

 I was weighing by equal arms, resistance to that amount. If 

 a greater amount of resistance is to be measured, I can help 

 myself by altering the first two branches of the resistance, 

 which represent the arms of the balance. If, by means of 

 stoppers, I make the resistance of one arm one hundred times 

 greater than the resistance of the other arm, then the resistance 

 I am seeking will be one hundred times greater than the other, 

 which I shall have to establish by stoppering. On the other 

 hand, if we want to measure very accurately a small resistance, 

 we alter the comparative resistances in the other sense, making 

 the stoppered value represent ten or a hundred times the resist- 

 ance to be measured. When we come, however, to measure the 

 leakage through a mile of cable, we should find not ten thousand 

 units, or one hundred thousand units, we should find one hun- 

 dred millions, or even as much as five hundred million units of 

 resistance. 



SINE METHOD. In such a case we must resort to a totally 

 different method, which we call the sine method. Here is an 

 instrument which is used for that purpose, being a very delicate 

 sine galvanometer. If a current flows through the coils of this 

 instrument, deflection of the needle takes place ; and this deflec- 

 tion increases with the current in the ratio of the sine of the 

 angle, provided you turn the instrument upon its vertical axis 

 through the angle of deflection. This needle being suspended 

 by a silk thread, and being made very astatic, an exceedingly 

 small amount of electricity flowing through, will produce notable 

 deflection. I have to connect my wire in this way, that I bring 

 my battery to one end of the copper conductor enclosed in the 

 insulating tube of gutta-percha or india-rubber, the other end 

 being sealed, and the external covering, or the water in which 



