Motion picture projection : an elementary text book (1928)

MOTION PICTURE PROJECTION 33 Electricity behaves in the electric circuit much like an incompressible fluid in a pipe line. We are very sure that electricity is not like any material substance which we know, but the common practice among students and shopmen of calling it "juice" shows that they think of it as like a fluid. We will, then, imagine the electric current to be a stream of electricity flowing around the circuit. One way of measuring the rapidity with which water is flowing is to let it pass through a meter which registers the total number of quarts or gallons which pass through. By dividing the quantity by the time it has taken to pass we obtain the rapidity of flow. There are instruments by means of which it is possible to measure the total quantity of electricity which passes any point in the circuit during a certain time. If we divide this quantity by the time, we obtain the amount of electricity which has passed in one second. This is a measure of the current strength. In practical work, however, the strength of the current is measured by instruments (ammeters) which show at each moment just how strong the current is, in somewhat the same manner as we may estimate the swiftness of a stream by watching a chip on the surface. This kind of an instrument enables us to tell at a glance what the current is without the necessity for a long experiment, and further we may detect changes in the strength of the current from moment to moment. In this connection it will be remembered that two measuring instruments are to be found on an automobile. The speedometer shows what the speed of the car is at each moment, so that the