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GROUNDS: HOW TO TEST FOR THEM 53
contacts the metal of the lamp frame or lamphouse. The negative wire has no such contact, hence the only effect is to charge metal with EMF— and there may be current loss through the permanent ground of the opposite wire, the amount of current flowing through the ground varying with the resistance set up by the earth at that particular point. The permanent ground of the projector frame is not considered.
(8) Sometimes opposite polarities have no contact, earth or otherwise, but carbon dust may settle across the carbon jaw, thus setting up a slight current leak. Or opposite polarity may contact metal, but if the contact is spotty because of dirt or other interfering element, the resistance will be high and little current will pass through.
If the ground has very low resistance it is called a "short circuit."
Testing For Grounds
(9) Testing for grounds is a very simple matter, once ^ the action of current is understood. If one polarity is grounded to your lamp or lamphouse (we, for the moment, disregard the Edison 3-wire system) and you touch one wire of a test lamp, or of a bell and battery to the lamphouse metal or lamp metal, and the other to a charged wire of opposite polarity, (10) it is evident current will flow through the ground contact and light the lamp or ring the bell, as the case may be.
(11) In making tests with a test lamp or bell and battery, the fact that the bell does not ring or the lamp does not light is not conclusive proof there is no ground. There^may be a ground of such high resistance that there is insufficient current to ring the bell or light the lamp. In that case hold one wire in contact with a live wire of opposite polarity and with the other make and break contact with the metal of the lamp or lamphouse, watching closely for a spark. This test should be made in darkness. (12) It is very important that all high resistance grounds be detected to prevent small but constant current loss.