128 POPULAR LECTURES AND ADDRESSES. 



to measure with the potential galvanometer. The 

 servant in every house that is lighted electrically 

 knows about potentials ; and if in reading the 

 galvanometer he sees it is down to eighty volts 

 he knows that something is wrong, and will at 

 once go to the engine-room and cause eighty-four 

 volts to be supplied ; supposing, for example (as 

 in the case of my own house, temporarily, until 

 I can get two-hundred-volt lamps), that the proper 

 potential is eighty-four volts. But in the current 

 galvanometer there are so many divisions indicat- 

 ing, it may be, the number of amperes in the 

 current. But after all, what do we want besides 

 a knowledge of the potential ? It is the sum of 

 the reciprocals of the resistances in the circuit. 

 In the multiple-arc system each fresh lamp lighted 

 adds a conductivity. In a circuit of Edison or 

 Swan hundred-volt lamps, in each of which you 

 have a current of 07 of an ampere, and therefore 

 a resistance of 143 ohms, how convenient it would 

 be, in putting on a lamp adding a certain con- 

 ductivity if we could say we add a mho, or a 

 fraction of a mho, as the case may be. I do not 



