The Ampère.—Now, in comparing the pounds pressure of water with the volts of pressure of electricity we used as an illustration a tank of water containing 100 gallons, and we saw that this water had a downward force or pressure in pounds. Let us now see what this pressure was acting upon.
It was forcing the quantity of water to spout upward through the end of the pipe. Now, as the quantity of water was 100 gallons, it could not all be forced at once out of the end of the pipe. The pounds pressure of water acting on the 100 gallons would force it out at a certain rate, which, let us say, would be one gallon per minute.
This would be the rate of the flow of water out of the tank.
Thus, you see, we find a second measurement to be considered in discharging the water-tank. The first was the force, or pounds of pressure, and the second the rate at which the quantity of water was being discharged per minute by that pressure.
This second measurement teaches us that a certain quantity will pass out of the pipe in a certain time if the pressure is steady, such quantity depending, of course, on the size or friction resistance of the pipe.
In electricity the volts of pressure act so as to force the quantity of current to flow through the wires at a certain rate per second, and the rate at which it flows is measured in ampères. For instance, let us suppose that an electric lamp required a pressure of 100 volts and a current of one ampère to light it up, we should have to supply a current of electricity flowing at the rate of one ampère, acted upon by an electromotive force of 100 volts.
You will see, therefore, that while the volt does not represent any electricity, but only its pressure, the ampère represents the rate of flow of the current itself.
You should remember that there are several words sometimes used in connection with the word "ampère"—for instance, we might say that a lamp required a "current" of one ampère or that a dynamo would give a "quantity" of 20 ampères.
The Ohm.—You have learned that the pressure would discharge the quantity of water at a certain rate through the pipe. Now, suppose we were to fix two discharge-pipes to the tank, the water would run away very much quicker, would it not? If we try to find a reason for this, we shall see that a pipe can only, at a given pressure, admit so much water through it at a time.