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THE POPULAR SCIENCE MONTHLY. 



which inventors have worked for this purpose : The first, which has 

 been used in every laboratory ever since electricity has been under- 

 stood, is the chemical method. When electricity passes through a 

 salt solution, it carries metal with it. and deposits it on the plate by 

 which the electricity leaves the liquid. The amount of metal depos- 

 ited is a measure of the quantity of electricity. Mr. Sprague and 

 Mr. Edison have adopted this method ; but, as it is impossible to allow 

 the whole of a strong current to pass through a liquid, the current is 

 divided ; a small proportion only is allowed to pass through. Provided 

 that the proportion does not vary, and that the metal never has any 

 motions on its own account, the increase in the weight of one of the 

 metal plates measures the quantity of electricity. 



The next method depends on the use of some sort of integrating 

 machine, and this, being the most obvious method, has been attempted 

 by a large number of inventors. Any machine of this kind is sure to 

 go, and is sure to indicate something, which will be more nearly a 

 measure of the electricity as the skill of the inventor is greater. 



Meters for electricity of the third class are dynamical in their 

 action, and I believe that what I have called the vibrating meter was 

 the first of its class. It is well known that a current passing round 

 iron makes it magnetic. The force which such a magnet exerts is 

 greater when the current is greater, but it is not simply proportional ; 

 if the current is twice or three times as strong, the force is four times 

 or nine times as great ; or, generally, the force is proportional to the 

 square of the current. Again, when a body vibrates under the influ- 

 ence of a controlling force, as a pendulum under the influence of 

 gravity, four times as much force is necessary to make it vibrate twice 

 as fast, and nine times to make it vibrate three times as fast ; or, gen- 

 erally, the square of the number measures the force. I will illustrate 



this by a model. Here are two 

 sticks nicely balanced on points, 

 and drawn into a middle position 

 by pieces of tape to which weights 

 may be hung. They ai*e identical 

 in every respect. I will now hang 

 a one-pound weight to each tape, 

 and let the pieces of wood swing. 

 They keep time together absolute- 

 ly. I will now put two pounds on 

 one tape. It is clear that the cor- 

 responding stick is going faster, 

 but certainly not twice as fast. I 

 will now hang on four pounds. 

 FlG ' 5 ' One stick is going at exactly twice 



the pace of the other. To make one go three times as fast, it is 

 obviously useless to put on three pounds, for it takes four to make 



