Permeameters of many different types, each having its special application, have been devised, but it is unnecessary to describe these in detail. Mention may, however, be made of a method, due to Drysdale, for testing magnetic qualities in bulk. A special drill is used, which cuts a hole with a tapering upper part and leaves a small projecting pin of the metal along the axis (fig. 20).
The pin is of standard diame ter, usually about O in. Into the hole is inserted a special closely fitting plug which carries the magnetizing and search coils, which surround the pin. Ballistic tests are carried out in the usual way. The method is quick, but for reliable results the drill and plug must be made to gauge very accurately.
The central neck is wound with a known number of turns of wire forming the B search coil, and outside this, separated by a known air space, is a second coil with a known number of turns. The
bobbin with the coils can be turned end for end through a semi circle. The change in flux through the two coils can be measured ballistically in the usual way. The difference in the change of flux through the two coils gives a measure of H. Specimens are some times made in the form of turned rods which fit into holes in the axes of the conical pole pieces. When the specimen is rotated, hysteresis loops cannot be taken. These may be determined by keeping the specimen stationary and changing the magnetizing current. It is difficult to make measurements in fields greater than 5,000 gauss by this method, owing to sparkling and to the time-lag in the magnetization of the iron cores and yokes of pow erful electromagnets. insteaci of turning the specimen, Weiss has measured the induction by withdrawing the test rod through a hole in the pole, the search coil being fixed.