All wires have a certain resistance per foot, according to the nature of the metal used and the size of the wire--that is to say, the finer the wire the greater number of ohms resistance it has to the foot. Water and electricity flow under very similar conditions--that is to say, each of them must have a channel, or conductor, and each of them requires pressure to force it onward. Water, however, being a tangible substance, requires a hollow conductor; while electricity, being intangible, will flow through a solid conductor. The iron of the water-pipe and the insulation of the electric wire serve the same purpose--namely, that of serving to prevent escape by reason of the pressure exerted. There is another term which should be mentioned in connection with resistance, as they are closely related, and that is _opposition_. There is no general electrical term of this name, but, as it will be most easily understood from the meaning of the word itself, we have used it. Let us give an example of what opposition would mean if applied to water. Probably every one knows that a water-wheel is a wheel having large blades, or "paddles," around its circumference. When the water, in trying to force its passage, rushes against one of these paddles it meets with its opposition, but overcomes it by pushing the paddle away. This brings around more opposition in the shape of another paddle, which the water also pushes away. And so this goes on, the water overcoming this opposition and turning the wheel around, by which means we can get water to do useful work for us. You must remember, however, that it is only by putting opposition in the path of a pressure and quantity of water that we can get this work. The same principle holds good in electricity. We make electricity in different ways, and in order to obtain useful work we put in its path the instruments, lamps, or machines which offer the proper amount of resistance, or opposition, to its passage, and thus obtain from this wonderful agent the work we desire to have done. You have learned that three important measurements in electricity are as follows: The _volt_ is the practical unit of measurement of _pressure_; The _ampère_ is the practical unit of measurement of the _rate of flow_; and The _ohm_ is the practical unit of measurement of _resistance_. III MAGNETISM Now we will try to explain to you something about magnets and magnetism. There are very few boys who have not seen and played with the ordinary magnets, shaped like a horseshoe, which are sold in all toy-stores as well as by those who sell electrical goods.