i6o LOUIS BELL 



ical and business like basis. Arc lighting, too, was proceeding 

 on broader lines with larger units, and presently came the 

 successful constant potential arc, putting it in the power of 

 the incandescent lighting station to do all classes of business 

 over the one network. This unification was perhaps the most 

 important step that has been taken in central station practice, 

 and it immediately gave electric lighting an economic lift that 

 was very quickly felt. It became possible to put prices at a 

 point that attracted busmess, and in the early nineties not 

 only was building active, but there was an extensive course 

 of rebuilding and change of equipment which really inaugu- 

 rated the modern period of electric lighting. 



The idea of unification of service was extended to alter- 

 nating current practice by the introduction of polyphase 

 apparatus. The first applications were to power transmission 

 plants, which did not in themselves affect the lighting industry 

 very greatly; but a little later the very extensive adoption of 

 polyphase transmission in central station working via sub- 

 stations rose to great importance in increasing the facilities 

 of the large urban plants. At the present time more than 60 

 per cent of the total dynamo capacity in the central stations 

 of the country is in the form of polyphase and other alternating 

 current generators. The introduction of these big modern 

 alternators has enabled conditions of good regulation to be 

 maintained on alternating current distribution systems, and 

 has, by improving the service, greatly stimulated incandescent 

 lighting on such systems. The substitution of large trans- 

 formers feeding secondary mains for the house to house dis- 

 tribution has also had a most important effect in improving 

 business and in increasing station economy so as to give more 

 encouraging financial returns. 



Arc lighting was much less stimulated by these changes 

 than was incandescent lighting, for the alternating arc has 

 never obtained a firm hold upon public popularity. Never- 

 theless, about 30 per cent of all the arc lights are to-day 

 operated by alternating current, a proportion which is due to 

 secondary rather than to primary causes. The incandescent 

 lamp has undergone in these years a far less extensive evolu- 

 tion than the arc lamp. It is now pretty nearly the same 



