Yet another type which must not be ignored is the 'Right-angled' pattern (Fig. 30), a name again self-descriptive. The horizontal carbon is the positive, and the vertical the
negative, and this lamp again is made by several manufacturers in slightly different forms.
This pattern lamp is in my experience the best of all for small currents, say, of 5 ampères or so, but inferior to Fig. 26 for currents of 10 ampères or more. This last remark perhaps hardly applies to alternating currents, which, however, I have not yet discussed. I cannot conclude this brief category of arc lamps without referring to the enclosed pattern, of which the 'Westminster' is perhaps the best-known and most popular (Fig. 31).
This is a lamp of the right-angled type, but the arc burns in a cylindrical glass chamber, not air-tight, but partially so. After burning a few minutes the oxygen in this chamber becomes used up and its place is taken by carbonic-acid gas and other products of combustion, after which the carbons burn away very much more slowly, and therefore require feeding at much greater intervals.
This lamp again is chiefly made for small currents not exceeding 5 ampères (and can therefore be used from any ordinary lamp socket), and for a moderate-sized hall is on the
whole as cheap, efficient and simple a lamp as any I am acquainted with. It can be supplied with or without mechanical centring movements as required, and is usually sent out with its own resistance for the particular current on which it is to be used, so that it only requires connecting up to the nearest lamp socket, and is ready for use.
It is not sufficient for anything larger than a 12-foot sheet or for working at a greater distance than, say, 40 feet, but within these limits the lamp, and in fact any good 5-ampère arc lamp, will be found quite satisfactory and saves the expense of putting in a special cable.
Automatic Arc Lamps.—Arc lamps for lantern work in which the feeding is done automatically are also made. Like hand-fed lamps, they vary in exact design, but all, or practically all, are so designed that the carbons are brought together by means of springs or weights, and some form of 'brake' controlled by a system of electro-magnets checks the