1 64 



NATURE 



\Jimc 14, 188; 



and it so happens that this alternating action exactly suits the 

 integrator. Suppose, however, that the action whatever it may 

 be, which we wish to estimate is of a continuous kind, such for 

 instance as the continuous passage of an electric current. Then, 

 if by means of any device, we can suitably incline the wheel, so 

 long as we keep pushing the cylinder along, so long will its 

 rotation measure and indicate the result ; but there must come a 

 time when the end of the cylinder is reached. If then we drag 

 it back again, instead of going on adding up, it will begin to 

 take off from the result, and the hands on the dial will go back- 

 wards, which is clearly wrong. So long as the current continues, 

 so long must the hands on the dial turn in one direction. This 

 effect is obtained in the instrument now on the table, the electric 

 energy meter, in this way. Clockwork causes the cylinder to 

 travel backwards and Forwards by m'a-is of what is called a 



Fig. 4. 



mangle motion, but instead of moving always in contact with 

 one wheel, the cylinder goes forward in contact with one, and 

 back in contact with another on its opposite side. In this in- 

 strument the inclination of the wheels is effected by an arrange- 

 ment of coils of wire, the main current passing through two fixed 

 concentric solenoids, and a shunt current through a great length 

 of fine wire on a movable solenoid, hanging in the space between 

 the others. The movable portion has an equal number of turns 

 in opposite directions, and is therefore unaffected by magnets 

 held near it. The effect of this arrangement is that the energy 

 of the current, that is, the quantity multiplied by the force driving 

 it, or the electrical equivalent of mechanical power, is measured 

 by the slope of the wheels, and the amount of work done by 

 the current during any time, by the number of turns of the 

 cylinder, which are registered on a dial. Professors Ayrton and 



Fig. 5. 



Perry have devised an instrument which is intended to show the 

 ^arae thing. They make use of a clook, and cause it to go too 

 fast or too slow by the action of the main on the shunt current ; 

 the amount of wrongness of the clock, and not the time shown, 

 is said to measure the work done by the current. This method 

 of measuring the electricity by the work it has done is one which 

 has been proposed to enable the electrical companies to make out 

 their bills. 



The other method is to measure the amount of electricity that 

 lias passed without regard to the work done. There are three 

 lines on which inventors have worked for this purpose. The 

 first, which has been used in every laboratory ever since elec- 

 tricity has been understood, is the chemical method. When 



electricity passes through a salt solution, it carries metal with it, 

 and dep >sits it on the plate by which the electricity leaves the 

 liquid. The amount of metal deposited 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 inte- 

 grating machine, and this being the mo.-t obvi >us method, has 

 heen 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 dynamic il in their 

 action, and I helieve that what I have called the vibrating meter 

 was the fir.-.t of its class. It is well known that a current passing 

 round iron makes it magnetic. The force which such a magnet 

 exerls 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 influence 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 generally 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 are identical in every respect. I will now hang a I lb. 

 weight to each tape, and let the pieces of wood suing. They 



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"-»- 



Fig. 6. 



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 4 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 g >es twice while the second goes three times. If instead 

 of a weight we use electromagnetic force to control the vibra- 

 tions of a bidy, then twice the current produces four times the 

 force, four times the force produces twice the rate ; three times 

 the current 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 Com- 

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

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

 will a'k 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. Now 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 vibra- 

 tion of the meter corresponds 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 has passed. Just as the 

 vibrating sticks in the model in time come to rest, so the 

 vibrating part of the meter would in time do the same, if it 

 were not kept going by an impulse automatically given to it 



