METERS FOR POWER AND ELECTRICITY. 369 



piston first one way then the other, overcomes resistance, and does 

 work. To find this, we must multiply the pressure by the motion at 

 every instant and add all the products together. This is what the 

 engine-power meter does, and it shows the continuously growing result 

 on a dial. When the piston moves, it drags the cylinder along ; where 

 the steam presses, the wheel is inclined. Neither action alone causes 

 the cylinder to turn, but when they occur together the cylinder turns, 

 and the number of turns registered on a dial shows with mathematical 

 accuracy how much work has been done. 



In the steam-engine work is done in an alternating manner, 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 be- 

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

 ward, 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 : Clock-work causes the cylinder to travel backward and 

 forward by means of what is called a 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 instrument the inclination of the wheels is effected by 

 an arrangement 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 equiva- 

 lent 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 num- 

 ber of turns of the cylinder, which is registered on a dial. Professors 

 Ayrton and Perry have devised an instrument which is intended to 

 show the same thing. They make use of a clock, 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 has 

 passed, without regard to the work done. There are three lines on 



TOL. XXIII. 24 



