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HANDBOOK OF KIN E M A T 0 GR A PHY. 159 FIG. 128. generator. When the prong is in the jaws of the switch contact current can flow along an unbroken metallic path. Pull the prong out and the track is interrupted. Fig. 128. Beside providing a copper track for elecricity to run round, there is another matter which is to be attended to in order that the current, especially current of the higher poten- tials, may be employed in safety. This is to see that the conducting circuit is surrounded by substances through which the current has a particular objection to flowing—otherwise called non-conductors of electricity. Non-conductors are non-metallic substances such as india-rubber, manilla, oiled cotton, silk, vulcanized rubber, paper, etc. Electric wires are insulated with such substances to make them safe to handle while ' alive,' or carrying electricity, also to prevent current from running to waste through chance metallic side tracks. Damp surfaces are -also partial conductors of electricity, and where a bare wire comes against such some of the current has a habit of trickling down into the ground below our feet and disappearing. This is known as an ' earth.' Where there is a doubt that electricity bills are unduly high, ' earths ' or ' grounds ' should be tested for by the electrician' using an insulation tester known as an ' ohmmeter and generator/ or ' megger.' Below will be found diagrammatic illustration of a direct and alter- nating current electric generator. The most fundamental difference between them is that whereas in the direct current generator or dynamo the commu- tator is segmented, in the alternator the commutator is of the ' split ring ' type. If, instead of driv- ing a dynamo by means of mechanical power, electric current is sent through its windings in the opposite direction to FIG. 129. DIRECT AND ALTERNATING GENERATOR COUPLED. which it would be given out in the usual way, the armature rotates of itself as a consequence. Thus, we may either use such an electro- magnetic machine to make electricity (for which purpose the armature is turned mechanically by say a steam, gas, or petrol engine), or we may use it as a source of mechanical energy by turning electricity into its coils.