Now let us see how the battery and wire are placed in connection with these instruments. You have seen that we usually have two wires for the electricity to travel in, one wire for it to leave the battery, and the other to return on. But you will easily see that if two wires had to be used in telegraphing it would be a very expensive matter, especially when they had to be carried thousands of miles. So, instead of using a second wire, we use the earth to carry back the electricity to the battery, because the earth is a better conductor even than wire. Although a quantity of ground equal in size to the wire would offer thousands of times greater resistance than the wire, yet, owing to the great body of our earth, its total resistance is even less than any telegraph wire used. When two electric wires are run from a battery and connected together through some instrument, this is called a "circuit," because the electricity has a path in which it can travel back to the battery. This would be a "metallic" circuit; _but when one wire only_ is used, and the other side of the battery is connected with the earth, it is called a "ground" or "earth" circuit, because the electricity returns through the earth. [Illustration: Fig. 8] If you look at this sketch (Fig. 8) you will see how the telegraph instruments are connected and will then be able to understand how a message can be sent. Here we have two sets of telegraph apparatus, one of which, let us say, is in New York and the other in Philadelphia. You will see that one wire from the battery is connected with the earth, and the other wire with the sounder. Another wire goes from the sounder to one leg of the key so as to make the brass base of the key part of the circuit. The other leg of the key is "insulated" from the brass base by being separated therefrom with some substance which will not carry electricity, such, for instance, as hard rubber. We will suppose that there is already a wire strung up on poles between New York and Philadelphia, and that the key, sounder, and battery in the latter city are connected in the same way as those in New York. Now, to enable us to send a message from one city to the other we must connect the ends of the wires to the instruments in each city; so we connect one end to the insulated leg of the key in New York, and the other end to the insulated leg of the key in Philadelphia. Everything is now completed, and, as soon as we find out what is the use of that part of the key that has a little round, black handle, we shall be ready to start. This is called the "switch."