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THE LIGHT SOURCE 369
savers," having come into wide-spread use, and effect true economy. One type is illustrated on Fig. 146. The process of using them is as follows : A cap is slipped on the rear (unburned) end of the stub. The other end of the cap then is shoved into the recess in upper end of handle c. The end of the stub is then rested on the wooden block, which must be of soft wood to prevent breaking the crater edges, and the upper end of the handle is struck a sharp blow with the palm of the hand. This not only shoves the cap tightly on the carbon end, but clamps it there immovably and in good electrical contact. The handle then is pulled off and the cap inserted in a recess in the end of rod d, and a knurled nut at its bottom is turned clockwise, which clamps the cap tightly into the rod. The rod in turn is inserted into the carbon jaws of the arc lamp. Such devices save as much as 25 per cent of the carbon.
Another way to provide for maximum use of carbons is to use specially processed carbons having a hole drilled into one end, and a pin milled at the other to fit into the hole. When a carbon is burned down several inches, a new carbon can be attached to it by this male-and-female means, the stub forming the tip of the new carbon ; and over the joint is placed a copper sleeve, which is comparable to the regular copper coating and burns away.
Figure 147
Mushroom Points
(41) What is known as a "mushroom" carbon tip is illustrated in Fig. 147, in which a mushroom appears on a positive (at the top) and a negative (at the bottom).