418 Prof. Ayrton and Mr. Medley on 



apparently improved; for using a spark of about 8 millimetres 

 in length, the lamps when new were entirely filled with a 

 bright blue glow accompanied with some bright white patches 

 on the glass ; whereas, after a run of about 80 hours at a 

 pressure of 100 volts, this phosphorescent effect with the 

 8 millimetre spark was either entirely absent, or consisted of 

 only a faint nebulous glow which did not fill the lamp-bulb, 

 and there was very little light on the surface of the glass. 



Thus it appeared as if Mr. Howell's explanation of the 

 great increase in the light given out by a glow -lamp during 

 the early portion of its life was correct. 



But this conclusion which we arrived at last year has been 

 modified by tests that we have been carrying out on Edison- 

 Swan glow-lamps purchased since our paper was sent in to 

 the Physical Society in November 1894. For we find that, — 

 while the induction-coil vacuum-test applied to these lamps 

 when they are new, and after a run of 80 hours at the normal 

 pressure of 100 volts, shows that the vacuum apparently 

 improves, — this improvement in the vacuum is not accom- 

 panied with the great rise in candle-power which we found 

 with all the Edison-Swan lamps purchased in 1892 and 1893, 

 the tests of which form the subject of this paper. And yet 

 the lamps are all marked in the same way, " 100 E.F. 8," 

 and they all look alike. 



In the case of a set of these Edison-Swan lamps, purchased 

 on January 21, 1895, not merely were the current and the 

 light periodically measured, but the induction-coil vacuum- 

 test was applied at intervals during the 80 hours' run ; and 

 it was found that the change in the phosphorescent appear- 

 ance of the lamp, and the consequent apparent increase in 

 the vacuum, was produced slowly and not suddenly on the 

 first application of the pressure of 100 volts. 



The following are the results of the tests of this batch of six 

 lamps. The light was measured with a Harcourt-Pentane 

 Standard, the potential difference with the Ayrton and Mather 

 reflecting electrostatic voltmeter, and the current with the 

 strip of manganin shunted with the Ayrton and Mather 

 moving coil-galvanometer ; this voltmeter and the ammeter 

 combination being the same as those employed with all the 

 lamps tested previously. But both instruments were carefully 

 calibrated on January of this year, to make quite certain that 

 the different behaviour of these newer lamps was not merely 

 an apparent one produeed by some change having occurred 

 in the sensibilities of the measuring-instruments. 



The insulation of the lamp-holders was carefully cleaned 

 and the contact-surfaces scraped before these tests were made. 



