The primary coil for the Oudin comprises about ten turns of No. 6 bare brass or copper wire, spacing the turns about 3/4" between centers. This coil is wound around a wooden form or cage as shown in the drawings. Strong spring clips, either home-made or purchased, are used to connect the condenser and spark-gap terminals with the Oudin primary; these clips are changed in position to connect in different numbers of turns while “tuning up” the Oudin coil for the maximum spark. The leads connecting the spark gap to the condenser to the Oudin coil, etc., should all be good heavy wire or, better still, heavy stranded copper cable. They may be made of copper strip which is used for switchboard winding, or flexible woven copper “pigtail” stock, measuring about 1" by 1/16" or more. The two high-voltage leads from the secondary of the step-up transformer to the condenser need not be of such heavy wire. Note particularly that, when connecting up the Oudin secondary to the primary coil, the lower end of the secondary winding is to be connected to that end of the primary coil; so that the current in the two windings when traced will always flow in the same or continuous direction.

Operation hints: In tuning up the Oudin coil “exciter circuit” the length of the spark can be increased by adjusting the number of turns, or fractions of a turn of the primary coil in the circuit by means of the clips mentioned; also by trying different capacities, or a different number of condenser plates; and again by varying the length of the spark gap or gaps. It is also advisable to have a rheostat in series with the spark-gap motor, so as to vary the speed of the motor, although this is not so important as the other factors.

A word of caution to the operator: Do not touch the 10- to 15,000-volt secondary winding of the step-up transformer, as it may prove disastrous; also be careful not to stand on or touch any “grounded” metal plate (such as a house heating register) when you hold a piece of wire or metal in your hand and attempt to draw sparks from the ball atop the Oudin coil.

The primary circuit of the 110-volt step-up transformer should have a “kick-back” preventer shunted across it, as shown in the diagram; this device is composed of two 1/2 mf. fixed condensers of 600 to 800 volts rating, with two small spark gaps shunted across them, the center connections of the gaps and condensers being grounded.

For demonstration purposes, plain glass vacuum tubes two or three feet long, which the demonstrator can wave about through the air and hold in various positions about the Oudin coil when in operation, will prove very entertaining to the audience. Sparks may be drawn into the body by holding a piece of wire in the hand.

A very good demonstration vacuum tube, 18 inches or more in length can be made up by tying a number of incandescent lamp bulbs along a small piece of bakelite or bamboo; every other bulb being reversed, so that a connection is established between the abutting bases, while the glass “tip” ends of the bulbs are connected by cementing a strip of tin-foil around the glass and joining the two strips of foil with a piece of wire.