119 
1906-7.] Photo-Electric Discharge from Platinum. 
centrated sulphuric acid, through the apparatus during the whole period of 
the research. The spark terminals T T were connected to the secondary 
of an induction coil, I (see fig. 3), used as a simple transformer, 
alternating current of periodicity 100 per second, supplied from an 
alternator on the premises, being used to excite the primary of the coil 
after transforming down to give a root mean square voltage of 45. Four 
large Leyden jars, L, of total capacity 0012 microfarad, were placed in 
parallel with the spark gap. 
The voltage on the primary of the induction coil and the periodicity of 
the current being kept always constant to within 1 or 2 per cent., the 
intensity of the ultra-violet light obtained was always the same to within 
the same degree of accuracy, and remained so for weeks without the slightest 
change in the intensity being detectable. A control “leak apparatus,” 
specially set up for the purpose of testing the constancy of the illumination 
during the course of the various sets of observations, proved to be quite 
unnecessary, and was only used, therefore, as an occasional check. This 
constancy had the great advantage of enabling readings taken at any time 
to be compared with confidence with those taken days or even weeks before. 
Owing to the difficulty of obtaining the regular use of the alternator, 
we tried various other sources of ultra-violet light, including a powerful 
Nernst lantern lamp, recommended by Mr H. S. Allen, but all proved quite 
