Suppose that, instead of emptying the water-tank from the roof through the pipe, we had just turned the tank over and let the water all pour out at once down to the ground. That would dispose of the water very quickly and by a short way, would it not? That is very easy to be seen, because there would be _no resistance_ to its passage to the ground. Well, suppose we had an electric battery giving a certain quantity of current, say five ampères, and we should take a large wire that would offer no resistance to that quantity and put it from one side of the battery to the other, a large current would flow at once and tend to exhaust the battery. This is called a _short circuit_ because there is little or no resistance, and it provides the current with an easy path to escape. Remember this, that _electricity always takes the easiest path_. It will take as many paths as are offered, but the largest quantity will always take the easiest. As the subject of resistance is one of the most important in electricity, we will give you one more example, because if you can obtain a good understanding of this principle it will help you to comprehend the whole subject more easily in your future studies. We started by comparison with a tank holding 100 gallons of water, discharging through a half-inch pipe, and showed you that the pounds of pressure would force the quantity of gallons through the pipe. When the tap was first opened the water would spout up very high, but as the water in the tank became lower the pressure would be less, and, consequently, the water would not spout so high. So, if it were desired to keep the water spouting up to the height it started with, we should have to keep the tank full, so as to have the same pounds of pressure all the time. But, if we wanted the water to spout still higher we should have to use other means, such as a force-pump, to obtain a greater pressure. Now, if we should use too many pounds pressure it would force the quantity of water more rapidly through the pipe and would cause the water to become heated because of the resistance of the pipe to the passage of that quantity acted upon by so great a pressure. This is just the same in electricity, except that the wire itself would become heated, some of the electricity being turned into heat and lost. If a wire were too small for the volts pressure and ampères of current of electricity the resistance of such wire would be overcome, and it would become red-hot and perhaps melt. Electricians are therefore very careful to calculate the resistance of the wires they use before putting them up, especially when they are for electric lighting, in order to make allowances for the ampères of current to flow through them, so that but little of the electricity will be turned into heat and thus rendered useless for their purpose. The unit of resistance is called the _ohm_ (pronounced like "home" without the "h").