It is a simple and easy matter to stop the flow of water from an ordinary faucet by placing your finger over the opening. As the water cannot then flow, your finger is what we will call a non-conductor and the water will be retained in the pipe. We have just the same effects in electricity. If we place some substance which is practically a non-conductor, or insulator, such as rubber, around an electric wire, or in the path of an electric current, the electricity, acted upon by the volts of pressure, cannot escape, because the insulation keeps it from doing so, just as the iron of the pipe keeps the water from escaping. Thus, you see, the volt does not itself represent electricity, but only the pressure which forces it through the wire. There are other words and expressions in electricity which are sometimes used in connection with the word "volt." These words are "pressure" and "intensity." We might say, for instance, that a certain dynamo machine had an electromotive force of 110 volts; or that the intensity of a cell of a battery was 2 volts, etc. We might mention, as another analogy, the pressure of steam in a boiler, which is measured or calculated in pounds, just as the pressure of water is measured. So, we might say that 100 pounds steam pressure used through the medium of a steam-engine to drive a dynamo could thus be changed to electricity at 100 volts pressure. _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.