THE CONQUEST OF TIME AND SPACE 



ment was iron oxide, with potassium hydrate as the 

 electrolyte. This battery showed no bad effects from 

 over-charging or from being rapidly discharged, but it 

 was found that the graphite soon became oxidized and 

 interfered with the working of the battery. This defect 

 was corrected by substituting chemically pure nickel for 

 the graphite, but another was soon discovered. Under 

 the pressure of the oxide of nickel the square tubes con- 

 taining the nickel were frequently injured so that the 

 powdered nickel oxide was sifted down on the pure 

 nickel layers and insulated them. 



The only solution of this difficulty seemed to be to 

 pack the nickel in strong round tubes four inches long 

 and about the size of a lead pencil, the sides of the tubes 

 being finely perforated. But the expense of producing 

 such tubes by ordinary methods was prohibitive. A 

 machine was finally invented, however, which made the 

 tubes economically by using spirally wound ribbons of 

 metal, the edges being fastened together during the coil- 

 ing process. By the use of these tubes the battery was 

 so far perfected that it was given extensive trials in 1908 

 on electric vehicles ; and as these tests proved satisfactory, 

 Mr. Edison began the construction of a specially de- 

 signed street car equipped with two 5-horse-power no- 

 volt motors of very light construction. The car weighs 

 complete about five tons, and the batteries are stored 

 under the seats running along each side. 



This car was tested continuously for three weeks on 

 one of the New York cross-town lines and performed its 

 work so satisfactorily and economically that the manage- 

 ment of the line decided to give the system a permanent 



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