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54
RICHARDSON'S BLUEBOOK OF PROJECTION
(13) If the system is an Edison 3-wire, a test lamp will not show light from neutral to ground because the neutral is already permanently grounded.
(14) When using an Edison 3-wire system it is safest to disconnect the wires by opening a switch, and test with either a battery and bell or with a magneto.
(15) A magneto is an excellent investment, because it generates high voltage and will immediately detect light grounds that would not be detected by either a battery and bell or a test lamp except by very careful work.
Figure 14
(16) Fig. 14 illustrates a convenient test lamp connection for a 110-220 volt test; one lamp and wires A C for the 110 volt test.
(17) To test with battery and bell, or with battery without any bell, first disconnect the mechanism from the circuit, either by opening a switch or by disconnecting the circuit wires. Next, connect two or three dry cells in series, with test wires leading from the outer poles of the battery thus formed to the parts to be tested. (18) Voltage from a single battery cell is too low to have value except in a very low resistance ground.
It is now only necessary to contact the two test wires across the suspected parts. If there is a ground, either the bell will ring or there will be a spark when the connection is broken. We prefer the spark test, for reasons already explained. (19) If an arc lamp is being tested, contact one carbon arm on "live" side of insulation, and the lamp frame with the other wire. If there is no bell