Common Science

by Carleton Washburne

Fig. 123. Electricity flows around a completed circuit somewhat as water might be made to flow around this trough.

Breaking and making the circuit. The most convenient way to put an air partition into an electric circuit and so to break it, or to close the circuit again so it will be complete, is to use a switch.

Experiment 67. In the laboratory, examine the three different kinds of switches where the electricity flows into the lamp and resistance wire and then out again. Trace the path the electricity must take from the wire coming into the building down to the first switch that it meets; then from one end of the wire through the brass or copper to which the wire is screwed, through the switch and on out into the end of the next piece of wire. Turn the first switch off and see how a partition of air is made between the place where the electricity comes in and the place where it would get out if it could. Turn the switch on and notice how this gives the electricity a complete path through to the next piece of wire. In this way follow the circuit on through all the switches to the electric lamp.

If you examine the socket into which the lamp screws and examine the lamp itself, you will see that electricity which goes to the outer part of the socket passes into the rim of the lamp; from here it goes into one end of the filament. It passes through the filament to the other end, which is connected to the little brass disk at the end of the lamp. From this you can see that it goes into the center point of the socket, and then on into the second wire that connects to the socket. Trace the current on back through this other wire until you see where this wire leads toward the dynamo. You should understand that the electric lamp, the switches, the fuses, all things along the circuit, are simply parts of the long loop from the dynamo, as shown in Figure 124.

Connecting in parallel. The trouble with Figure 124 is that it is a little too simple. From looking at it you might think that the loop entered only one building. And it might seem that turning off one switch would shut off the electricity all along the line. It would, too, if the circuit were arranged exactly as shown above. To avoid this, and for other reasons, the main loop from the dynamo has branches so that the electricity can go through any or all of them at the same time and so that shutting off one branch will not affect the others. Electricians call this connecting in parallel; there are many parallel circuits from one power house.

Fig. 124. Diagram of the complete circuit through the laboratory switches.

Figure 125 illustrates the principle just explained. As there diagrammed, the electricity passes out from the dynamo along the lower wire and goes down the left-hand wire of circuit A through one of the electric lamps that is turned on, and then it goes back through the right-hand wire of the A circuit to the upper wire of the main circuit and then on back to the dynamo. But only a part of the electricity goes through the A circuit; part goes on to the B circuit, and there it passes partly through the electric iron. Then it goes back through the other wire to the dynamo. No electricity can get through the electric lamp on the B circuit, because the switch to the lamp is open. The switch on the C circuit is open; so no electricity can pass through it.

The purpose of the diagram is to show that electricity from the dynamo may go through several branch circuits and then get back to the dynamo, and that shutting off the electricity from one branch circuit does not shut it off from the others. And the purpose of this section is to make it clear that electricity can flow only through a complete circuit; it must have an unbroken path from the dynamo back to the dynamo again or from one pole of the battery back to the other pole. If the electricity does not have a complete circuit, it will not flow.

Application 52. A small boy disconnected the doorbell batteries from the wires that ran to them, and when he wanted to put the wires back, he could not remember how they had been connected. He tried fastening both wires to the carbon part of the battery, connecting one wire to the carbon and one to the zinc, and connecting both to the zinc. Then he decided that one wire was all that had to be connected anyway, that the second was simply to make it stronger. Which of the ways he tried, if any, would have been right?