118 Proceedings of the Koyal Society of Edinburgh. [Sess. 
light, whereas a mercury vapour arc lamp is extremely suitable. But the 
difficulty arises from the fact that the lamp must be placed between the 
poles of an electro-magnet, the distance between the poles being usually so 
small that any tilting apparatus that may be employed interferes with the 
proper mounting of the lamp. 
We have recently overcome this difficulty by constructing a lamp 
which may be fixed in position between the poles of an electro-magnet, or 
in any other suitable position, and then started without having to be tilted. 
This is brought about by the employment of a heating arrangement near 
one of the electrodes, preferably the negative electrode. The lamp tube is 
provided with a small vessel near this electrode, the vessel having a re- 
entrant portion or recess in which a heating element is placed. The part 
of the tube immediately above this small vessel and its recess is con- 
stricted. The heating element may conveniently consist of a small coil of 
platinum wire wound round a suitable support; it may be placed in the 
recess of the small vessel or removed at will, without interfering with the 
vacuum of the lamp. The heating coil of wire is connected in series 
or in shunt, in the latter case being provided with an automatic 
cut-out. An external resistance is placed in series with the lamp. Before 
starting, the small vessel is full of mercury, which forms a continuous 
connection inside the tube between the positive and negative electrodes. 
When the electrical current is switched on, the heating coil becomes 
incandescent, and the heat given off by the wire goes to raising 
the temperature of the vessel and its contained mercury, there being no 
appreciable loss by radiation into the surrounding air. Very little heat is 
required, because the first bubble of mercury vapour formed rises to the 
constricted portion of the lamp tube and is there caught, thus breaking the 
continuity of the mercury inside the lamp and starting the arc. Owing 
to the resistance of the mercury vapour, which is formed once the arc is 
started, the current is cut down to the value required for running the lamp. 
The platinum wire of the heating element can be made of such a thickness, 
and the external resistance can be so adjusted, that the wire does not emit 
heat when the lamp is working, but becomes incandescent when the lamp 
is started, the action being quite automatic. 
The construction and method of working the lamp may be more par- 
ticularly described with the help of the accompanying diagrams. Fig. 1 
represents a vertical section of the small vessel A, with the heating 
element G, shown in elevation, in position. This figure shows one method 
of fitting in the heating element. B is the recess in the vessel A, and C 
represents the constricted portion of the lamp tube. Through a part of the 
