Insulating Tube.—The successful operation of an induction coil without breaking down when under strain depends largely upon the insulating tube which separates the primary and secondary. Hard rubber tubes are perhaps the best. A tube may be easily built up of several layers of 1/2-inch sheet hard rubber by steaming it so as to soften it and then wrapping it around a form. The tube should fit the primary tightly and be about one inch shorter than the core. After the tube is in place it is poured full of beeswax and rosin in order to fill all interstices and prevent sparks due to the condenser effect of the windings from jumping from the inside of the tube to the primary.
Secondary.—A coil used as a wireless telegraph transmitter must have wire of large cross section in its secondary so as to obtain a heavy disruptive discharge. Numbers 34 and 32 B. S. are generally used for small coils and numbers 30 and 28 B. S. for large coils. Silk covered wire is the usual practice, but enameled wire is coming into use. Cotton covered wire takes up too much space and has poorer insulating qualities.
Enameled wire is insulated by a coating of cellulose acetate, which has a dielectric strength of about twice that of cotton and takes up much less room than silk. There is, then, with enameled wire a great saving in space, and a greater number of turns may be placed on the secondary without increasing its mean distance from the core. The following table shows the comparative diameters of silk and enamel covered wires suitable in size for use on the secondaries of induction coils.
In winding enameled wire it must be taken into consideration that the insulation of enameled wire is rigid and has no give. Consequently, to allow for expansion, enameled wire must be more loosely wound than fiber or silk covered wire. The occasional insertion of a layer of paper in winding will give room for expansion and at the same time not add greatly to the diameter.
Fig. 27. Theoretical and practical form of secondary.
The length of the secondary is generally not much more than one-half the length of the core. Coils giving sparks up to 2 inches in length may be wound in two sections or in layer windings, but the layer winding is not recommended for coils giving sparks over one inch. It is best in a coil of this kind to insert an occasional layer of paper. The paper should be well shellacked or paraffined and be of a good grade of linen. It should project about one-quarter of an inch from the ends of the secondary as shown by the sectional drawing in Fig. 28.
This insertion of paper increases the insulation and renders the liability of sparks jumping from layer to layer much less, as is the case when the layers are very long.
