398 Prof. Ayrton and Mr. Medley on 



of glow-lamp a certain number of hours could be experi- 

 mentally determined at the end of which it would be economical 

 to discard lamps even when the price of a Board of Trade 

 unit was as low as 4J<i. ; secondly, that it was only economical 

 to run lamps at the pressure marked on them for one particular 

 price of electric energy. 



As Edison-Swan lamps were the only lamps that could be 

 legally used in this country in 1892, and as a pressure 

 of 100 volts was one very commonly employed for electric 

 lighting, we decided to experimentally find out what was the 

 most economical pressure to maintain between the terminals 

 of nominal 100-volt 8-candle lamps of this make, and what 

 was the length of their economic lives for different prices of 

 a Board of Trade unit. 



As it is well known that the lives of filaments are consider- 

 ably affected by the steadiness of the pressure between their 

 terminals, it was decided to run the lamps from a battery 

 of storage-cells, and to arrange that the pressure when on the 

 lamps should not vary by as much as one tenth per cent. 

 The capacity, however, of the cells at the Central Technical 

 College which could be entirely set on one side for this 

 investigation and used for running 100-volt lamps from five 

 o'clock every evening to nine or ten o'clock the next morn- 

 ing for five nights of the week, only allowed of nine 8 C.P. 

 lamps being dealt with at a time. The nine lamps were 

 divided into three groups, each containing three lamps, and 

 a perfectly constant, but different P.D. was maintained 

 between the terminals of the lamps in each group. 



When our experiments were begun it was known that the 

 price of lamps would fall in the autumn of 1893, therefore it 

 was considered hardly probable that any economy would be 

 gained by running the lamps at less than their normal 

 pressure. Hence we decided to run the three groups at 100, 

 102, and 104 volts respectively. The tests, however, soon 

 proved that no economy could be gained by running these 

 lamps at as high a pressure as that of 104 volts, and so the 

 group running at that pressure was changed for one running 

 at 101 volts. 



The pressure of the storage-cells which supplied the lamps 

 with current diminished, of course, during the night, so that 

 it was necessary to introduce between the cells and the lamps 

 a variable resistance which could be altered to keep the 

 pressure on the lamps constant within one tenth per cent., 

 and to avoid constant attention it was also necessary that 

 this resistance should be automatically controlled. 



As no automatic regulator could be purchased which would 



