TCBA Volume 12 - Issue 1
Page 6 of 18
A 24-inch Oudin High-Frequency Coil
With regard to the type of spark gap to be used, this may be of the ordinary stationary type, using a set of cooling fins fitting tightly onto the zinc or other spark-gap electrodes. One of the drawings shows how to build such a spark gap. With power as great as 1 K.W. (1,000 watts) however, and providing the coil is left operating for any length of time, these gaps heat up and arc; and it is best to employ a rotary spark gap to keep the sparkling surfaces cool.
Another drawing shows how to build a rotary spark gap, an ordinary fan or other motor driving the circular disc, which contains about 12 zinc or brass plugs firmly secured around the edge of the disc. These may be screwed into place against a brass or copper band around the edge of the insulating hard-rubber or bakelite disc, which may be screwed to the metal with four or more screws in the manner shown. The two stationary electrodes for the rotary gap can be made in the same manner as the drawing shows for a stationary gap; the rods supporting the electrodes are slid through the upright support rods and held at any desired position by means of the clamp screws. They may be 8-32 (or 10-32) threaded rods, fitted with insulating handles as shown in the sketch, which permits a far more accurate adjustment of the length of the spark gaps.
The construction of the Oudin coil does not involve any great cost for materials, for if you have some wire about No. 26 gauge, either bare, enameled, or single-cotton-covered, this will do.
Because of the voltage, which may reach a half million or more, the turns of wire on the Oudin secondary have to be spaced about the thickness of the wire apart, in order to prevent a breakdown between the turns. There are several ways to do this: one of the simplest being to wind between turns string or thread the same size as the wire; the thread being removed after the winding is completed. You could do the same thing by winding a second wire along with the one that is to remain on the coil after removing the second wire. Still another way is to make a loop of a piece of the same wire used in winding the coil, and to hang a weight on the bottom of the loop so that it automatically spaces the turns as you proceed to wind the coil.
A secondary form as large as 11 inches in diameter and 51 inches long is not easy to pick up, but it can be made up very simply as shown in the drawing or in any other manner which the constructor may invent. The plan illustrated calls for four or, better, six wood discs DD, over which are fastened a series of thin wood slats. These slats may be bradded to the discs; but it is much better to glue and dowel them, to avoid having any metal at all in the coil form, on account of the tremendous voltages which will be induced in the coil wound on this form. The secondary may be supported in the manner shown in one of the drawings; the tube fitting down over a 2" or 3" thick circular wooden plug, doweled fast to the base.
After the secondary wood form has been built up, it can be wrapped with several layers of any suitable insulating material such as paper, cardboard, fiber, oiled linen, micanite, etc.
After the form has thoroughly dried, (some shellac or glue having been used to cement the layers of paper) the form may be wound with about 850 to 1000 turns of No. 26 copper wire. The simplest way to wind the coil will be to build a couple of supports as shown in the drawing, one person guiding the wire while the other one turns the crank. The shaft to turn this large coil had probably best be a square wood stick or piece of iron, the square wood turning freely in round holes bored in the uprights, as shown in the detail sketches. It is not desirable to shellac the finished secondary coil, because of the high capacity effect of the shellac. Also, if it is not thoroughly dry by the time the coil is put into operation, it will cause electric leakage between the turns. The polished brass ball is mounted on a brass or iron rod about 10 inches high at the upper end of the Oudin secondary, and may be about three inches in diameter.