Newman N. Holland

While the electric telephone is probably the most wonderful instrument that has ever been produced, it is so simple in construction that it can be readily made without the necessity of having many tools or any great expense. By following instructions here given, the amateur can construct a telephone set that will give practically as good service as those in commercial use.

A telephone set consists of really three separate instruments, the transmitter, the receiver, and the signalling bell. We will take up each part separately, commencing with the transmitter, and explain how they can be made with little difficulty and with parts easily procured.

The Transmitter.

The first thing to procure is the casing or box to hold the parts, and an old cigar box (Puritanos size) is admirably suited for the purpose. This box should be first cleaned of all paper labels, etc., and brass hinges substituted for the cloth one usually employed. In the exact centre of the corner of this box a hole should be cut about 7/8" in diameter. This hole should have its edge bevelled, which can readily be done with a pocket knife, and a piece of pasteboard F, glued inside so as to form a bell shaped funnel such as is shown in the drawing. On the inside of the cover should be pasted a thick cardboard ring H, concentric with the opening. This ring can be readily cut out by means of a compass and a pair of scissors, and should be about 1-8" thick and 1/4" rim, the outside circumference being 21/2". An important part of the transmitter is the diaphragm E. This can be made of a piece of smooth tin or ferrotype plate such as is used in taking tintypes, and can be cut out with an ordinary pair of large scissors. The diameter of this should be the same as the paper washer refered to (21/2"). Before cutting the dia phragm the circle should be struck out with a pair of compasses and the centre marked; through this centre marking, a hole should be made that will allow a 4-32 screw to freely pass. At equal distance around the circumference and 1/8" from the edge, four other holes should also be made in the diaphragm so that it can be screwed to the box: make these holes same size as that in the centre, It is now necessary to procure two pieces of carbon which are called respectively the back and front electrode. The front electrode C, is fastened directly on the diaphragm and consists of a circular disk 1/8" thick and 11/8" diameter. The centre of this disk is also bored for 4-32 screw and is se-scured to the diaphragm by passing a screw of that size 1/4" long through both carbon and diaphragm, having the head of the screw next the cardboard and using a small brass nut to hold it to the diaphragm.

The next step is to construct a bridge to hold the back electrode D, in its proper position. This may be made of wood. Two blocks should be accurately made of the following dimensions to form posts to hold the cross piece, cut to 3/4 " x 1/4" x 1/4". Next make a strip of wood 31/2" long x 1/2" wide x 1/8" thick. Fasten the two posts which you have already made at the extreme ends of this strip by means of two 1/2" No. 4 round head wood screws in each block. In the bridge thus made, a hole is to be bored in the exact centre of the cross piece.

This hole should be large enough to allow an 8-32 machine screw to pass through it.

We now come to probably the most difficult part of the instrument for the amateur to make. This is the back electrode, and should consist of a carbon block " in thickness and 11/8" in diameter.

How To Make A Telephone I The Transmitter 311

Fig. 1.

The centre of this block is bored to a depth of 1/4" with 3/8" drill, and the hole continued through with a No. 18 drill. If the reader is in possesion of a small lathe, it is preferable that the face of this disk be scored with a number of concentric ridges as illustrated in the drawing, but while such construction is much better it is not absolutely essential for the working of the instrument. After the back electrode has been made it should be surrounded with a piece of cloth, allowing the edges to project around the front side about 1/4". Almost any kind of woolen cloth will do for this purpose, but it should not be too thick or stiff. The cloth can be glued to the carbon and then should be further secured by winding some stout thread around the outside a number of times quite tightly, and then tying the thread securely. The portion of the cloth which has been allowed to project beyond the outer surface of the carbon should now be frayed by pulling out the threads which run around parallel to the face of the carbon. This will give a light fringe sticking up beyond the carbon, and this fringe should be given a tendency to flare outwardly by pressing the carbon block, fringe end down, onto a smooth surface and seeing that all the threads of the cloth are projected outwardly.

The carbon block is now already to screw on the wooden bridge already constructed. This should be secured by passing an 8-32" screw through the carbon block and the hole in the centre of the bridge, binding them together with a nut in a manner very similar to that which we employed with the front electrode. The screw in the back, however, should be at least 1" long as we will use it also to form one of the terminals for connection, and should carry an additional nut beside that used to hold it to the bridge.

We are now ready to assemble our instrument, and although it is necessary to put some granulated carbon between the back and front electrodes, it is better to assemble the instrument first without the carbon to get each part in proper position and then take it apart when such positions have been correctly found and insert the necessary amount of carbon granules.

The bridge with its back electrode should be laid so as to span the diaphragm we have already mounted on the box and adjusted until back and front electrode are exactly opposite one another and the fringe of the cloth fully covers the front electrode. When this position is obtained, carefully mark the position of the posts with a pencil mark all around them, and then after removing same, in each of the spaces obtained drill two holes in such a manner as to allow of putting through two No. 4 wood screws. The bridge can then be replaced carefully and the wooden screws inserted from the front side of the cover.

After the instrument has been assembled in this manner, the four screws holding the bridge to the cover of the box should be removed and the cover itself taken from the box by unscrewing the hinges. It is then ready for inserting the granulated carbon. The granulated carbon should be of the size which is called No. 40 mesh; that is, the particles should be of a size that will just go through a screen having forty holes to the inch. This carbon should be purchased, rather than attempt to make it, and should your local electric supply man not have any in stock, he can doubtless readily procure for you what you require.

Now to fill the transmitter with carbon, the back electrode mounted on its bridge should be held with fringe side up so as to form a shallow cup. This cup should be two-thirds filled with the granulated carbon, that is, so that the carbon grains will cover about two-thirds of both electrodes when the instrument is assembled and held in the position in which it will be used.