1912-13.] New Method of Starting Mercury Vapour Apparatus. 119 
electrode D is sealed the leading-in wire E, the outer end of which is con- 
nected to the metallic cap F. The heating element G consists of a hollow 
quartz tube H, round the tapering end of which is wound a coil of thin 
platinum wire I, one end of which is connected to the metallic sleeve K. 
The other end of the wire passes down through the tube and is connected 
to the flexible conductor L, which is held in position by a packing of 
plaster-of-Paris M, or other suitable cement. A metallic clip N, having a 
binding screw O, serves to connect the sleeve K and the cap F, and also 
holds the heating element G in position. Figs. 2 and 3 are cross-sections 
taken on the lines XX, YY (fig. 1) respectively. The heating element G 
may be removed, whenever desired, by loosening the screw O. It is advis- 
able to wind a thin piece of asbestos thread round the outside of the 
tapering portion of the heating element, or to adopt some similar pre- 
caution, in order to prevent the platinum wire, when hot, from coming into 
direct contact with the glass wall of the recess B of the small vessel A. 
Fig. 4 shows, in elevation, the end of the lamp tube, and illustrates a 
second method of mounting the heating element, a vertical section of which 
is shown in fig. 5. This alternative arrangement is shown, partially in 
vertical section, in fig. 6, while fig. 7 is a horizontal section taken on the 
line ZZ (fig. 6). Referring to figs. 4, 5, 6, and 7 : A represents the small 
vessel with the recess B and the constricted portion C of the lamp tube. D 
is the electrode, and the leading-in wire E is connected to a bayonet-holder 
P, which is attached to the vessel A by means of cement R. The heating 
element consists of a hollow porcelain tube H with fluted end, the coil of 
platinum wire I being attached as in the previous case. The metallic 
sleeve K is provided with two pins SS, which fit into two slots in the 
bayonet-holder P. The heating element may therefore be placed in position 
or removed with the greatest of ease. 
In fig. 8 (Plate) is shown a convenient form of the complete lamp as used 
for spectroscopic purposes. For use in connection with work on the Zeeman 
effect, the lamp tube may be of the form shown, when it is to be placed in 
a vertical position between the poles of the electro-magnet, or it may be 
provided with a very short horizontal portion, so that it may be fixed up in 
such a position that the light passes in the direction of the lines of force. 
This does away with the necessity of boring the pole pieces, except where 
the light is observed in a direction parallel to the lines of force. 
The method of working the lamp is as follows : — The flexible conductor 
L and the other electrode of the lamp than that shown in figs. 1-7 are 
connected through a suitable resistance to the poles of the battery. When 
the lamp is not burning, there is a continuous column of mercury between 
