Electromotive force may be created in many ways, and time will not permit us to refer to these in detail, but it must be taken that electrical machines, batteries, and dynamos are all of them appliances for creating electromotive force, or, as it is sometimes called, electric pressure, just as various kinds of force-pumps are contrivances for creating pressure in fluids. We find that electromotive force acts differently on various substances when they are subjected to its operation. In some substances electromotive force produces a continuous electric current, and in these cases the material is called a _conductor_. In other cases electromotive force creates what is called _electric strain_, or electric displacement, and these substances are generally called _non-conductors_. The difference between conductors and non-conductors can be illustrated by a mechanical analogy. Consider, for instance, a force-pump consisting of a cylinder with a tightly fitted piston; suppose the bottom of the pump-tube to be closed by a pipe having in it a tap. If we open the tap and apply pressure to the piston, we can force out of the pipe a current of air which continues to flow as long as the piston is being pressed down. In this case the pressure on the piston corresponds to an electromotive force, and the current of air flowing out corresponds with the electric current in the electrical circuit. Supposing, however, that we shut the tap and then attempt to force down the piston, we find at once an elastic resistance to motion. The piston can be pressed down a little way, compressing the air and thus creating a strain; but if the pressure is removed the piston flies up again, on account of the compressional elasticity of the air. In this operation we have a mechanical illustration of the action of electromotive force on a material such as glass or air, which is called a non-conductor, or sometimes a dielectric. In these bodies electromotive force produces an electric strain, just as the mechanical force produces in the air enclosed in the cylinder a mechanical strain. When the tap at the bottom of the cylinder is closed, we can, by applying pressure, force down the piston a little way, but that movement cannot be continued, because we are building up an opposing pressure due to the elasticity of the air.