492 



Dr. H H. Poole on the Electrical 



the surface of the ebonite free from moisture. The only 

 trouble experienced with this contact-maker occurred at high 

 pressures, when the sparking which occurs at the brushes 

 ionized the air to such an extent that appreciable leakage 

 occurred through it. If this reached the low-pressure brush 

 it caused too high a reading of the galvanometer. If it only 

 occurred between the high-pressure brush and the earthed 

 frame of the motor it was less important,, but had the effect 



Fig. 2. 



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of rendering the pressure unsteady. By the use of suitably 

 placed vulcanite and metal screens the brushes were enclosed, 

 so that this trouble was practically eliminated for pressures 

 up to 10,000 volts. For larger pressures a larger apparatus 

 with longer air-gaps would be necessary, but, as 10,000 volts 

 greatly exceeded the pressure for which the valves were 

 designed, it was considered inexpedient to try and increase 

 the pressure further. 



