The advance of photography : its history and modern applications (1911)

RONTGEN-RAY PHOTOGRAPHY 343 varieties even greater changes than this can be brought about. The action of such a coil depends on two or three wellknown principles. One of these may be stated thus : If we have two circuits lying quite close to one another, but not in absolute contact, and a current of electricity be sent round one of these circuits, then, at the moment the circuit is completed an instantaneous current passes round the other circuit in the opposite direction, owing to what is called the inductive action of the first or primary current. The induced current is usually called the secondary current. Again, when the primary current is stopped, i.e. at the is moment when the circuit is broken, an induced current instantly excited in the secondary circuit in the same direction as the primary. Consequently, if arrangements are made for quickly closing and opening the primary circuit alternately, then an alternating current will be induced in the secondary circuit. In the Ruhmkorff coil the primary circuit passes round a core of soft iron, which, of course, becomes a magnet when the current is made, and loses its magnetism when the break of the current takes place. Outside this primary coil is a secondary coil which consists of many thousands of turns of fine wire all very carefully insulated. This wire is not wound in one layer from end to end of the coil, then another on the top of this and so on, but is in separate sections perpendicular to the axis of the coil, these sections being separated from one another by partitions which are made of very good insulating material. In this way it is possible to ensure that no two consecutive layers shall be at such a great difference of potential as to cause a breakdown in the insulation, and a consequent short circuiting due to sparking across from layer to layer. The primary current is obtained by a battery of a few accumulators, or from