Fig. 39.
In fact, when a rapid vibrator is being used with the coil, the leading wires from the secondary terminals present this brush appearance, the curious threads of light resembling luminous hairs waving in the air. The more rapid the vibrations the more prominent the brush effect, as will be seen in the Tesla coils. The positive ball of the discharger shows the brush as a spreading mass of luminous threads reaching out toward the negative ball, which latter resembles a star, as in the figure.
The intensely disruptive power of the long spark is readily shown by its power to perforate substances, but great care must be taken that the secondary wires of a coil are led away from the body of the coil. A good plan is to hang two silk cords or stout threads from the ceiling, to which the secondary wires may be attached and kept in sight when experimenting at any distance from the coil.
To pierce a piece of thin glass, take two lumps of paraffin about the size of a walnut, and, warming them and the glass sheet, stick them on opposite sides of the glass facing each other. Then warm the ends of the two pointed wires and thrust them into the lumps of paraffin, that they terminate on the glass surface directly opposite each other. On connecting these to the secondary coil a few impulses to the contact breaker will start an electric discharge sufficient to pierce the glass if the thickness be proportioned to the power of the apparatus. The great Spottiswood coil pierced a block of glass 6 inches in thickness.
There is, however, a certain element of danger to the secondary insulation in performing this experiment.
CHAPTER VI.
SPECTRUM ANALYSIS.
If a metal or the salt of a metal be burned in a flame it imparts to the flame a distinctive color; table salt thrown into the fire burns with a yellowish flame, denoting the presence of sodium, and a greenish tint, indicating the combustion of chlorine. Violet flames accompany the burning of the salts of potassium, and barium burns green. Lithium and strontium give a red hue. But to be ordinarily perceptible, the salts require for the most part to be present in considerable quantities. By the use of the spectroscope, however, extremely small proportions of these metals and salts can be readily detected and classified.
