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GROUNDS: HOW TO TEST FOR THEM 55
action or spark, all is well — so far as the low voltage of a battery will determine. A magneto is a better testing instrument. It detects not only an actual ground, but also discovers weaknesses in the insulation.
To test with test lamp, the same procedure is followed, except that the switch must be closed and the circuit tKus charged with line voltage.
Permanent Test Lamp
(20) It is well to install a test lamp permanently connected to ground. To do this it is only necessary to install a socket (two of them if a 3-wire test outfit as illustrated in Fig. 14 is required) on the projection room wall at any convenient place, connecting one side of it to ground by means of a water pipe, to which clean, firm, metallic contact must be made. To the other side connect two test wires long enough to reach the parts to be tested. The permanent ground wire should have a switch so that it may be disconnected. A trouble-lamp retrieving reel is excellent for these wires. It may be installed on the ceiling beside the test lamps. One reel will handle both wires, which may be ordinary lamp cord.
(21) If a ground to earth is suspected, use one wire — the one opposite to the permanent ground wire. (22) If you wish to test for the kind of ground that means a limited "short" between parts of an arc lamp, pull the switch, disconnecting the earth ground (disconnect projector ground wire) and use both wires on suspected parts.
Grounding for Projector
(23) The projector lamphouse and stand should always be connected to ground by means of a "ground wire/' which is a wire connecting the part with permanent grounds, such as water pipes, the metal work of the projection room, frame work, etc. This is to avoid shocks to the men at work. The motor generator frame and stand of spots, etc., should be similarly grounded.
(24) Projectionists should tesl their apparatus, par