If we have gradually mastered the principles of electricity we shall find their application not as difficult as it may have seemed at the start. The telephone is one of the most interesting instruments with which to experiment. In its simplest form it consists of a bar magnet, around one end of which is coiled a fine insulated copper wire; the ends of this coil of wire are attached to two binding screws or terminals, by means of which connection is made to transmitting wires; in front of the end of the magnet around which the wire coil is wound is a circular plate of very thin iron, gripped at its edges but free to vibrate at the center. Nothing more is essential to the construction of the telephone. To carry on a conversation two identical instruments may be employed. The sound waves of speech striking the disk of iron cause it to vibrate sufficiently to touch the magnet; these vibrations are carried by an electric current to the magnet at the other end, where these vibrations are repeated. It is interesting to notice that we may still keep our figure of speech by speaking of sound waves that induce electric or magnetic waves. To construct a telephone capable of transmitting to a distance of two or three miles we need a pair of square bar-magnets, about 3 in. long. These should be capable of sustaining each other if the marked end of the one be presented to the unmarked end of the other. In addition we need two small boxes, preferably round, about 2 1/2 in. in diameter and 1 1/2 in. deep, a pair of thin iron plates such as are used for "tin-types," two pairs of small binding screws, of any pattern, so long as they are small, a piece of cylindrical white wood-a thick broom handle would do-about 1 in. in diameter and 12 in. long, and one-fourth ounce of No. 36 silk covered copper wire. Let us first cut off two pieces of the wooden rod 4 1/4 in. in length, and with a sharp knife split each one down the middle. With a 1/4 in. chisel we cut grooves in the flat face of each half cylinder just deep enough to allow the magnets to lie between halves when these are fitted together. The grooves should extend the whole length of the rods, but at one end for a length of two inches the channels should be a trifle wider than the bar-magnets. The half cylinders must now be glued together so that we now have two cylinders with a square channel through the middle. Next we cut a round hole, of exactly the same diameter as the wooden cylinders just finished. in the center of the bottom of the little box. The top end of the cylinder (the end at which the groove is smaller) is then thrust into the bottom of the box from the outside, until it is just flush with the inside of the bottom of the box. A little glue should be used to make sure that the cylinder will not shift. Now in the center of the lid of the wooden boxes cut a circular hole 1 3/4 in. in diameter, then glue a cone of stiff pasteboard into this hole. The cone should open and extend outwards for about an inch and be cut off flush with the inside of the lid; its purpose is to collect and reenforce the sound waves created when we speak into the telephone. The next step is to cut two circles out of the tin-type plates of such a size as to fit exactly into the inside of the lids of the boxes. As any dents or kinks would destroy the usefulness of the plates extreme care must be used. Having measured the exact diameter of the inside of the lids, lay out a corresponding circle on a stout piece of cardboard; cut out this piece with scissors, and then, laying this cardboard piece over the iron plate, scratch lightly a line all around with the point of a pin. It is easy now, with sharp scissors, to cut the desired plate to size. An even simpler receiver can be made from an old baking powder tin-one-half pound size is large enough. Solder a piece of heavy wire on the inside about 1 1/4 in. from the bottom. If we have a piece of hard steel or piece of an old file we can make the magnet either by contact with another powerful magnet or by passing a strong current of electricity around it. One end of the core should now be wound with wire, just as was explained above. For short distance experiments no farther materials are needed; for longer distances it will be necessary to introduce an electro-magnet near the permanent magnet. The details of the many varieties do not interest us now. Put the magnet in its wooden cylinder, the iron disk in the round box so that the magnet is just clear. The magnet can easily be adjusted by sliding it in the grooves. In practice a gentle tap on the disk will tell whether or not it is touching the magnet-a clear ring shows it is free to vibrate; a dull thud that it is touching. The coil of the magnet should be at the further end from the disk. A disk of wood or stiff cardboard can be screwed or pasted to the end of the cylinder, holes cut through it to allow wires to be connected to the coil around the magnet. All that remains is to string wires between the desired points and connect the outside wires with the inside, as was done with the telegraph line. No boy will be satisfied with the telephone as here outlined. In an article of this kind there is no space to devote to making suggestions for finishing or polishing the wood, making brackets, etc., for the instrument, and the many fine points which will be developed. It will be easy to make the instrument look more like the Bell telephone in commercial use by using more expensive materials. Several dry cells connected with the magnets will help to make the experiment useful for greater distances. In short, there is almost no limit to the improvements which may be made.