56 On Recent Improvements in Electric Lighting. 



size of the wire on the armature and to place a considerably 

 greater resistance than usual on the coils, not by the use of 

 finer wire, but by a greater number of turns, and then only 

 allowing a shunted portion of the current from the armature 

 to pass through them. By this means he increased the 

 efficiency of the machine to a considerable extent, that all 

 danger of its becoming heated was avoided, and that any 

 irregularity in the driving engine did not affect the new 

 machine nearly as much as the old. The necessit}- of keep- 

 ing the machine cool whilst working has been recognised by 

 all makers as an absolute requirement. Apart from the fact 

 of the liability to injury by this heating, it has been esti- 

 mated that hitherto half the current developed in any 

 machine was only available for lighting purposes, the other 

 half being wasted in heating the machine and conducting 

 wires. Various mechanical devices have been used to this 

 purpose. In the earlier forms of Dynamo machines a stream 

 of water was kept flowing through the armature axis ; 

 in the Brush machine the iron of the armature has 

 grooves cut nearly through it in various places, so as 

 to have an insulating medium of air in various parts ; 

 and, later on, in the Weston machine the iron part of 

 the armature is built up of thin plates so as to leave 

 hollow spaces ; the axis is also hollow. This permits the air 

 to rush through and pass between the coils of wire, and so 

 tends to keep them from heating. 



I have up to this point treated of machines giving a 

 continuous current — that is, of the same polarity. 



Whilst continuous currents appear to be the most 

 efficient, they, of course, consume carbon points un- 

 equally, the carbon that is connected with the positive 

 pole of the machine being consumed at nearly double the 

 rate of that of the negative. (Latterly Dr. Siemens has 

 stated that the negative carbon only wastes away from 

 the constant heat, and suggests other substances that might 

 be kept cool for the negative pole.) 



For burning carbon candles, and for some systems 

 where more than one light is required to be main- 

 tained in one circuit, alternate current machines 

 are required. Many forms have lately been intro- 

 duced, and apparently with great success; the best 

 appear to be the new Gramme, the Lontin, and the 

 Siemens. All these alternate current machines require an 

 exciter for the electro-magnets, consisting usually of a small 



