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134 the machine is increased. The more efficient the machine, that is to say, the less of the energy passing through it is lost, the cooler will the machine run. Excessive heating in a machine is also objectionable on the ground that thereby some of the materials used in the construction may be destroyed.

If we have to deal with a dynamo machine this is especially important, since in the construction of such machine insulating materials such as cotton, tape, wood, etc., must be used. The machine should therefore be designed with due regard to a strictly limited temperature rise, and it is also important that the finished machine should be tested so as to make sure that the designer's intention has actually been realised. The heat is generated in the body of the materials used, and it has to leak out and be dissipated into the surrounding atmosphere from the surface of the machine. Thus it is quite possible that the temperature at the surface, which can be measured by a thermometer, is much below the internal temperature, just as the outer surface of a stove is not nearly so hot as the fire inside. To get the temperature of the hottest part, we should put a thermometer to the inside of the machine, but