Page:Electricity (1912) Kapp.djvu/203

Rh Before the current flows there is no magnetism in this system, or, to speak more correctly, there is but a very feeble magnetisation. It is next to impossible to have any piece of iron absolutely devoid of any trace of magnetism. The very act of machining the iron during the process of manufacture is sufficient to produce some feeble magnetisation. This fact the reader may test for himself in a very simple manner. Let him take an ordinary kitchen poker, hold it north-south and twist it with his hands. If he then approaches one end to a compass needle and then the other, he will find that the poker has become a feeble magnet. By reversing the position and twisting again he will be able to reverse the polarity, thus proving that it is the feeble influence of the earth’s magnetism which, combined with the mechanical stress on the molecules due to the twisting, has produced the magnetisation. The material of the field-magnet undergoes during the process of beig worked into shape a good deal of mechanical stressing, and hence becomes magnetic. There is thus, to start with, some feeble magnetisation in the system. The corresponding e.m.f. produced at the brushes is also correspondingly feeble, but if the