The heads of the primary drum are wooden rings, seven inches in diameter outside, four and one-half inches inside and one-half an inch thick. Six cross bars, two and one-half inches long, three-quarters of an inch thick and one-half an inch wide are required to support the wire. They are spaced equidistantly around the rings and held in position by means of brass screws passing through the rings. Do not use iron screws, because iron is magnetic and should be entirely avoided in the construction of a Tesla coil. Small notches should be cut in the outside edge of the cross bars to accommodate the wires. The wires should pass around the drum in the form of a spiral with one-quarter to five-sixteenths of an inch space between the turns. The completed drum will somewhat resemble a squirrel cage. The ends of the primary winding should terminate in two large binding posts mounted on the primary heads. The heads are fastened to the centre of the base by a couple of large wood screws passing upwards though the bottom. [Illustration: FIG. 177.—Diagram of connections for operating the Coil.] The illustration in Figure 177 shows how to connect the Tesla coil. The primary should be in series with a condenser and a spark gap. The condenser should consist of two or three Leyden jars or several glass plates coated with tinfoil. It is impossible to determine just how much capacity the condenser should have in advance, because the length of the conducting wires, adjustment of the spark gap, etc., will have considerable effect. The condenser is connected directly across the terminals of the spark coil. The spark gap may consist of two one-eighth inch brass rods supported by two double binding posts mounted on a small wooden block. The induction coil should be as large as possible. When the coil is set in operation it will charge the condenser and a white, snappy spark should pass across the gap. If the fingers are brought near to one of the secondary terminals of the Tesla coil, a small, reddish purple spark will jump out to meet them. It will be necessary to adjust the apparatus very carefully before a spark of any considerable length can be obtained. Changing the length of the spark gap and the size of the condenser will undoubtedly produce results. It may also be possible to lengthen the high frequency spark by disconnecting one of the wires from the primary binding posts on the Tesla coil and connecting the wire directly to some one of the primary turns. A very small change in some one of the connections may produce considerable result. The purpose of the adjustments is to tune the circuit in the same manner that a wireless outfit is tuned by altering the capacity of the condenser or varying the number of turns in the helix. [Illustration: FIG. 178.—Plate Glass Condenser.]