The paper must be put on smoothly and evenly, so as to afford a firm foundation for the next layer. The wire is wrapped around with the paper, so that the next layer starts one-half inch from the edge. A second layer is then wound on very carefully, stopping when it comes one-half inch from the edge. Two more layers of paper are put on, and the process repeated, alternately winding on paper and wire until the stated quantity of the latter has been used up. The layers of wire may occasionally be given a coating of shellac. This is a good insulator, and will serve to hold them together and prevent the wire from becoming loose. In winding the coil, remember that if at any point you allow the winding to become irregular or uneven, the irregularity will be much exaggerated on the succeeding layers. For this reason, do not allow any to occur. If the wire tends to go on unevenly, wrap an extra layer of thick paper around underneath so as to offer a smooth foundation, and you will find the difficulty remedied. [Illustration: Fig. 164.—Interrupter Parts.] An efficient vibrator for a coil cannot be easily made, and it is best to buy one which is already fitted with platinum points. The interrupter will play a very important part in the successful working of the coil, and its arrangement and construction are important. Interrupters like that shown in the illustration and used for automobile will be found the best. The condenser may be home-made. It consists of alternate sheets of tinfoil and paraffined paper, arranged in a pile as shown in the illustration. The following table gives the proper sizes for condensers for three different coils. ────────────────────────────────────────────────────────────────── TINFOIL SIZE OF SPARK-COIL ─────────────────────────────────────────── NO. SHEETS SIZE OF SHEETS ────────────────────────────────────────────────────────────────── 1/2 inch 50 2 x 2 ────────────────────────────────────────────────────────────────── 1 inch 100 7 x 5 ────────────────────────────────────────────────────────────────── 1 1/2 inch 100 8 x 6 ────────────────────────────────────────────────────────────────── The paper must be about one-half inch larger all the way around, so as to leave a good margin. The alternate sheets of tinfoil, that is, all on one side and all on the other, are connected. The condenser is connected directly across the terminals of the interrupter. [Illustration: Fig. 165.—Condenser.] There are various methods of mounting a coil, the most common being to place it in a box with the interrupter at one end. Perhaps, however, one of the neatest and also the simplest methods is to mount it in the manner shown in the illustration.