240 



Mr. C. Vernon Boys 



[March 2, 



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

 generally, the square of the number measures the force. I will illus- 

 trate 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 are identical in every respect. I will now hang 

 a 1 lb. weight to each tape, and let the pieces of wood swing. They 

 keep time together absolutely. I will now put 2 lbs. on one tape. It 

 is clear that the corresponding stick is going faster, but certainly not 

 twice as fast. I will now hang on 4 lbs. 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 3 lbs., for it takes four to make it go 

 twice as fast. I will hang on 9 lbs. One now goes exactly three times 

 as fast as the other. I will now put 4 lbs. on the first, and leave the 

 9 lbs. on the second ; the first goes twice while the second goes three 



times. If instead of a weight 

 Fig. 5. nv we use electro - magnetic 



force to control the vibra- 

 tions of a body, then twice 

 the current produces four 

 times the force, four times 

 the force produces twice the 

 rate : three times the cur- 

 rent produces nine times the 

 force, nine times the force 

 produces three times the 

 rate, and so on : or the rate 

 is directly proportional to 

 the current strength. There 

 is on the table a working 

 meter made on this principle. 

 I allow the current that 

 passes through to pass also 

 through a galvanometer of special construction, so that you can tell 

 by the position of a spot of light on a scale the strength of the current. 

 At the present time there is no current ; the light is on the zero of 

 the scale, the meter is at rest. I now allow a current to pass from a 

 battery of the new Faure-Sellon-Volckmar cells which the Storage 

 Company have kindly lent me for this occasion. The light moves 

 through one division on the scale, and the meter has started. I will 

 ask you to observe its rate of vibration. I will now double the 

 current ; this is indicated by the light moving to the end of the 

 second division on the scale : the meter vibrates twice as fast. Nov? 

 the current is three times as strong, now four times, and so on. You 

 will observe that the position of the spot of light and the rate of 

 vibration always correspond. Every vibration of the meter corre- 

 sponds to a definite quantity of electricity, and causes a hand on a 

 dial to move on one step. By looking at the dial, we can see how 

 many vibrations there have been, and therefore how much electricity 



