98 Electric Lighting. 



a very high amount of efficiency has been obtained, with 

 the use of less wire on the revolving coils than usual; this 

 extra efficiency being gained by the methods of arranging 

 the coils, and also by placing nearly the whole of the wire 

 in the magnetic field. Amongst the most prominent and 

 successful of these modifications are the Weston, Burgin, 

 and Schukert machines. The Weston has a Siemens arma- 

 ture, but contains half the number of coils on its circumfer- 

 ence, the place of the wire being taken by soft iron, and the 

 wire itself laying in a channel surrounded by soft iron, has 

 its inductive effects considerably augmented. The Burgin is 

 a combination of the Siemens and Gramme. The revolving 

 bobbin is long,like the Siemens, but it is built up of a series 

 of rings, wound something like the Gramme, and connected 

 together. The Schukert is a Gramme ring, but exposes 

 nearly the whole of the wire in the magnetic field. Dynamo- 

 machines are now required, giving two different kinds of 

 current — low tension for single, and high tension for multiple 

 arc lights. The low tension is of course easily attainable, 

 large wire and low speed, giving a very large current of very 

 low tension. The most remarkable description of machine 

 of this kind is Edison's, the armature being built up of bars 

 of copper, having practically no resistance whatever, driven 

 at a speed of 350 revolutions per minute, it is stated it will 

 supply sufficient current to maintain 1000 incandescent 

 lamps of about seven candle-power each, all joined up of 

 course in parallel circuit. 



To obtain currents of high tension is not so easy. With 

 the present system of generating currents, it is attained 

 by either decreasing the size of the wire on the bobbins 

 and increasing the number of convolutions, or by increasing 

 the speed of rotation. In the first instance the trouble 

 of heating and destroying the coils makes its appearance, 

 and in the second the difficulties of mechanical construc- 

 tion — that of making the revolving bobbin in such a form 

 as to be perfectly secure, and to prevent the wire flying 

 off when revolved at very high speeds. Consequently, up 

 to the present time, the highest tension current machine 

 made is that of Brush, capable of supporting forty arc lights 

 on one circuit. 



The armature is a modification of the Gramme ring, and 

 contains a very large quantity of wire wound in spaces cut 

 out of the iron ring. Special precautions are taken to pre- 

 vent cross-currents being generated, and for keeping the 



