will pass from one to the other. This phenomenon and others associated with it are most readily studied by the use of an electrical machine or an induction coil, between the electrodes of which a great difference of potential can be easily produced. If the spark be examined with the spectroscope, its spectrum is found to be characterized by lines which are due to the metals composing the electrodes, and to the medium between them.
The passage of the spark through air or any dielectric is attended with a sharp report, and if the dielectric be solid, it is perforated or ruptured. If the electrodes be separated by a considerable distance, the path of the spark is usually a zigzag one. It is probable that this is due to irregularities in the dielectric, due to the presence of dust particles.
With proper adjustment of the electrodes, the discharge may sometimes be made to take the form of a long brush springing from the positive electrode, with a single trunk which branches and becomes invisible before reaching the negative electrode. Accompanying this is usually a number of small and irregular brushes starting from the negative electrode.
Another form of discharge consists of a pale luminous glow covering part of the surface of one or both electrodes. If a small conducting body be interposed between the electrodes when the glow is established, a portion of the glow will be cut off, marking out a region on the electrode which is the projection of the intervening conductor by the lines of electrical force. This phenomenon is called the electrical shadow.
The difference of potential required to set up a spark between two slightly convex metallic surfaces, separated by a stratum of air 0.125 centimetre thick, has been shown by Thomson to be about 5500 volts. The difference of potential which produces the sparks between the electrodes of an electrical machine, which are sometimes fifty or sixty centimetres long, must therefore be very great. The quantity of electricity which passes during the discharge is, however, exceedingly small, on account of the great resistance of the medium through which the discharge takes place.
