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RONTGEN-RAY PHOTOGRAPHY
Introductory. — In these days of electric trams and trains , everybody is more or less familiar with the sparking which takes place, when, owing to the presence of dirt or some other cause of break in the electric circuit, the current is obliged to jump across a small air-gap. Now, the power of the electric current to complete a circuit in which the conductors are separated by an air gap depends upon what is known as the voltage, and not on the actual quantity of electricity which is passing through the circuit. Unless the voltage is very much higher than that used for lighting or traction purposes (about 200 volts), it requires but an extremely small gap in the circuit to cause the current to cease altogether. Thus, a voltage of over 30,000 is necessar}7 to enable it to jump across an air-gap of one inch. A method has long been known by means of which it is possible to obtain a relatively small current at a ver}^ high voltage, by the use of a few simple cells and a suitable transformer. This is very important, for in Rontgen-ray work it is necessary to have such an apparatus, since a simple trial will be sufficient to convince one that the voltage obtainable with a few accumulators in series, is of no use for sending a discharge through a vacuum tube.
The Buhmkorff Coil. — The transformer used for this purpose is what is familiarly known as a " coil/' or, more correctly, as an induction coil ; it is also sometimes spoken of as Ruhmkorff's coil. With the help of even cheap forms of this coil, we can obtain a voltage of some tens of thousands by using only a few accumulators in the primary circuit, while with larger and more expensive