X-Rays, Apparatus and Methods Page 4

the primary), but, being "short-circuited" by the primary, at once discharges about the iron core in a direction opposite to the regular "make" current, thus reducing (if correctly proportioned) the magnetic intensity of the iron core to zero. All this takes place so quickly that the practical effect of the action is merely to assist and intensify the dropping off of the core's magnetism — to shove it along — hasten it. Considering the secondary, now, we find that we shall have at break an induced discharge of greatly higher E.M.F. than that at make, owing to the falling off of the core's magnetism at break being greatly sharper than its growth at make. The effect of this is to give secondary discharges in one direction only at all times when the spark gap is not greatly shorter than the maximum obtainable spark gap for that particular condition of running the coil, the make induced current at such times being of too small E.M.F. to get across the gap at all. This is always the condition when X-ray tubes are used. A further function of the condenser is to suppress burning at the "break" terminals or points of interruption by taking up the energy of the magnetic discharge of the coil.

In the construction of the induction coil, a number of points may be noted. Our secondaries are wound in sections 1/2-inch thick (according to the plan originally proposed, we believe, by Ritchie). These sections are separated from one another by a large number of discs of paper of a brand especially selected as free from carbon particles, and baked at a temperature a little below charring point for some time immediately before use — this to drive out moisture. These sections are then assembled and immediately immersed in a special insulation composition having a very high melting point, with high specific heat, and some slight viscosity at all temperatures. Paraffine we absolutely prohibit as being apt to crack and absorb moisture. The melting point of paraffine, also, is so low (not over about 140° F. for the highest), that in warm weather there is serious risk of displacement of some of the sections by their own weight; for the same reason but little energy can be dissipated from the secondary.