On Recent Improvements in Electric Lighting. 55 



improved armature, which gave a considerable increase in 

 the power to the machines which were then constructed. 

 Then came Holmes, Gramme, and many others ; and whilst 

 it is true that in 1857 Holmes's machines were used in one or 

 two lighthouses, they did not come up to the necessary 

 standard required for electric lighting. 



In 1863 Wilde introduced his machine, which was a con- 

 siderable advance on all former; in the place of permanent 

 he used electro magnets, which were excited either by a 

 battery or small magneto machine. At this time, also, Ladd 

 and many others made machines of a similar description. In 

 1866-7 discoveries were announced by Wheatstone, Siemens, 

 and Varley simultaneously that a separate exciter for the 

 electro-magnets was unnecessary, and that it was sufficient to 

 pass the current induced in the armature through the coils of 

 wire surrounding the electro-magnets, which, provided they 

 were made of hard iron, retained a certain amount of magnet- 

 ism. Each revolution of the armature served to increase this 

 magnetism, until a maximum effect was obtained. Dating 

 from this period, an immense number of machines based on 

 this principle have been constructed, notably the Gramme, 

 Siemens, Brush, Wallace-Farmer, Maxim, Weston, and a host 

 of others, all differing only in minor details, or different 

 methods of constructing the armatures or connections with 

 the commutators. These machines, however, appear to have 

 vastly different degrees of efficiency, the Siemens, Brush, 

 and Gramme being accepted, I think, as giving the best 

 results. 



According to experiments formerly made, the maximum 

 effect was obtained by making the resistance of the wires of 

 the machine equal to the resistance of the outside circuit, 

 following the same law as the voltaic battery. 



Now, this rule has usually been carried out, but with this 

 disadvantage — if the resistance of the outside circuit happens 

 to fall lower (such as the carbons of the lamp remaining in 

 contact, &c.) the driving engine running at a higher speed a 

 larger amount of current was developed in the coils of the 

 electro-magnets, heating them, and thus tending to destroy 

 their insulation ; it was therefore advisable for safety to work 

 with a larger outside resistance, but with its attendant dis- 

 advantage of reducing the amount of current the machine 

 was capable of producing. 



In a paper recently read by Dr. Siemens before the Royal 

 Society he stated that he had found it better to increase the 



