INCANDESCENT GASES.
The gases for this purpose are enclosed in a tube of the shape shown by Fig. 765. The part c is intended to be placed parallel to the length of the slit. The ends A and B have platinum terminals with aluminium extensions inside, which are connected to the high voltage terminals of a. Ruhmkorff's induction coil, and the secondary current passed through the tube. The heating effect of this current makes the gas glow. The sealed projec tion, D, is where the air is exhausted by means of a mercury pump and the re quired gas allowed to enter. The pres sure of the gas should be stated on the outside of the tube, but is seldom given. This arrangement generally requires two to four secondary cells in series with the primary of the induction coil. The strength of the current in the tube, and therefore its consequent heating effect (varying approximately with the square of the current's magnitude), may be regu lated by tightening the adjusting screw belonging to the spring of the contact breaker, or by altering the current in the primary circuit. A Wimshurst induction
machine can be used instead of the coil ; this will necessitate a motor to keep it in continuous rotation. It should have a Leyden jar connected to each of its prime conductors, and the terminals of the Geissler tube should be attached to the outer coatings of the jars by silk-covered wires as shown by Fig. 766. These wires, as also those of the secondary circuit of the induction coil, should be kept well apart in air, and the stand holding the gas tube should be thoroughly insulated. Avoid getting near the wires while the current is flowing, or a. disagreeable shock may be received. Do not place the Geiss ler tubes too near the collimator when using large coils, as the spectrometer is generally made of metal throughout, and a shock might be experienced on ap proaching the eye to the instrument.