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CH. I] MAGIC LANTERN WITH DIRECT CURRENT 21
In testing the lantern the following points should be especially looked to :
(A) That there is voltage in the supply line. This is easily determined by turning on the incandescent lamp (fig. 2), or by trying to light the arc lamp.
(B) That the arc lamp is in working order and has carbons long enough to last during the exhibition. By closing the switch and bringing the carbons in contact and slightly separating them the arc light should be established almost instantly (see also § 30). It takes a certain amount of experience to tell whether the carbons are long enough to last during the exhibition. If there is any doubt, put a new pair in position.
From the high temperature of the carbons, and the lamp generally, after the current has been on some time, it is not easy to put in new carbons in the midst of a demonstration. It also makes an embarrassing break in the exercises (see § 27).
§ 27. Gloves with asbestos patches. — In spite of all precautions it is sometimes necessary to work about the arc lamp after it has been running, and is therefore very hot. By the use of suitable pliers or tongs one can usually manage to do the things necessary ; but for certainty and rapidity one always needs to be able to use the hands directly. This is rendered possible by the use of gloves with asbestos patches in the places which come in direct contact with the hot metal or carbons. The gauntlet form of gloves is best for then the wrists also are protected.
The asbestos patches may be of asbestos cloth, or preferably of quilted asbestos paper. The asbestos cloth is very thick and clumsy. The asbestos paper of about half a millimeter thickness (Vso in.) quilted between thin cotton or linen cloth answers well. The quilting stitches should be long and extend obliquely in two directions (fig. 5). The object of the quilting is to overcome the weakness and easy tearing of the asbestos paper.
For most work a patch on the thumb and index finger is sufficient but as it is often convenient to grasp a hot carbon between the index and middle finger, it is well to have a patch on the middle finger also (fig. 5).