620 SCIENCE PROGRESS 



A most interesting and probably very important recent 

 application of aluminium in the electrical industry, based upon 

 its electrical, physical and chemical properties, now claims more 

 than passing attention. 



The manufacture of coils, whether for motors, dynamos or 

 other electrical apparatus, involves the insulation of each turn 

 of wire from its neighbour so as to ensure that the current will 

 pass only along the path ordained for it. One of the greatest 

 problems which the designer of electrical machinery has to 

 face is to get a sufficient number of turns into the space at 

 disposal, which is usually very restricted. As has already 

 been shown, an aluminium wire has to be materially larger 

 than a copper wire, so that if it were necessary to insulate it 

 in the way practised in the case of copper wire (wrapping with 

 rubber, silk, etc.), the use of aluminium would be very dis- 

 advantageous. Aluminium, however, has a chemical property 

 which has been pressed into the service of the electrician in 

 a most ingenious manner. The surface of the metal is normally 

 covered by a thin, invisible coating of oxide. By immersing 

 the metal in suitable solutions, this oxide film can be increased 

 in thickness until it opposes sufficient resistance to the passage of 

 a current to prevent a short circuit between two adjoining turns 

 of wire in a coil. By this means it has been possible to make 

 coils for electrical machinery which according to Mariage 

 {Tramway and Railway World, May 4, 191 1) show a saving in 

 weight of about 50 per cent, (which owing to the position of 

 the coils in electrically propelled vehicles is a saving of very 

 great moment) and a reduction in cost of 60 per cent. More- 

 over, unlike the usual insulating material, being entirely 

 inorganic, the coating made on aluminium is improved rather 

 than damaged by heat, so that the danger of burning the 

 insulation and so short circuiting the coils is diminished. On 

 the other hand, the size of the coil must still be somewhat 

 larger and the difficulty of making effective joints is greater 

 than in the case of copper. Such coils have not been in use 

 very long but their application seems to be increasing very 

 rapidly and the writer is of the opinion that their ultimate 

 adoption on a very large scale is assured. 



Space does not allow of a detailed discussion of the use of 

 aluminium in other directions in the electrical industry and 

 mention can therefore only be made of such articles as current 



