ELECTRICAL CONFERENCE AT PHILADELPHIA 627 



magnetic system, and we connect these two systems by that great physi- 

 cal constant, the ratio of electro-magnetic to the electrostatic system of 

 units. 



What can be simpler in theory than the electrostatic system, based, 

 as it is, on the law that electric attraction varies inversely as the square 

 of the distance? We only have to know how the electricity is dis- 

 tributed and its attraction is known. Hence we must select the simplest 

 possible case, such as two parallel disks, and to render the problem cal- 

 culable, we add a guard ring to the movable disk. We then have the 

 absolute electrometer of Thomson. This gives us a measure of the 

 electric potential. Knowing the capacity and difference of potential 

 of the surface of a condenser, we know its charge. But all these quan- 

 tities, the calculation of the electrometer and the capacity of the con- 

 denser, depend upon the mathematical theory of electric distribution. 

 Are we able to calculate the capacity of condensers of all forms? I am 

 sorry to say we are not. The modern method of treatment is due to 

 George Green, an English investigator, whose name should be held in 

 honor by all electricians. But this method is what is called an inverse 

 one. It is not a method by which we can calculate the distribution 

 on any body at random, but the shape of the body and the electrical 

 distribution on it are both found at once by a species, as it were, of 

 exploration and discovery. So that we cannot make our electrometers 

 and condensers of any shape and then calculate them, but we are forced 

 to make them of some simple geometrical form whose solution is 

 known. We fit our apparatus to the mathematics rather than the mathe- 

 matics to the apparatus. 



But when we have satisfied all the conditions we measure out our 

 static charges as easily as a quantity of matter. The manufacturer sells 

 the oxygen and hydrogen in iron cylinders and determines the amount 

 by the product of the capacity of the cylinders by the pressure. Were 

 there any buyers of electricity we might sell them a Leyden jar full and 

 determine the amount by the product of the capacity of the jar by the 

 electric potential. According to this analogy, then, the electricity is 

 similar to matter and the potential fluid pressure, while the word ca- 

 pacity has a similar meaning in both. 



In the electro-magnetic method of electrical measurement we make 

 use of the magnetic action of the current, either on a neighboring mag- 

 net or another current or portion of the same current. The laws of the 

 action of a current on a magnet were discovered by Biot and Savart, 

 and of two currents on each other by Ampere, and the results applied to 



