1898 - 99 .] Rev. A. Moffat on Energy of the Rontgen Rays. 435 
'the slit lie deduces the duration of the emission of the Rontgen 
rays, and gives as the mean result 0 ‘00109 second. 
Now, with reference to all these experiments, it has to he 
observed that they were made with induction coils. A great 
difficulty in the way of using an induction coil for such an investi- 
gation is that, owing to the nature of the apparatus, each discharge 
given by a coil lasts for a certain time. This is not the case, or, 
at least, is so to a very much less extent with influence machines. 
The discharges from these machines being, because of the small 
capacity, practically instantaneous, experiments conducted with 
them give much clearer results. 
To get the whole duration of the discharge of the Rontgen rays in 
one second, we must multiply the duration of each discharge by the 
number of discharges. The method adopted for determining the 
number of discharges was that given long ago by Professor E. Wiede- 
mann (Wied. Ann., 10, p. 210). In the circuit containing the 
Rontgen lamp and its air-spark, the length of the spark being 14 
mm., a Geissler tube (H) was set upright outside the box. Parallel 
to it was fixed another Geissler tube (N) excited by an induction 
coil. The interruptions of the primary current in the coil were 
made by a vibrating tuning-fork which made 100 vibrations per 
second. By viewing the images of the Geissler tubes in a rotating 
mirror, one could easily determine the relation between the number 
of discharges in the tube H and those in the tube N.* The 
number so obtained was about 90. The whole duration of the 
emission of the Rontgen rays is, therefore, not more than 
90 x xo"o^ooo5 ToVo Rm whole time which elapses. 
My experiments gave the following results : — 
Let a = length of air-spark in millimetres. 
b — the extent to which the light from the Hefner lamp was 
weakened. 
c = distance in centimetres of the photometer from the paper 
in front of the Hefner lamp. 
* Between the bright images of the tube H, due to the discharges which 
gave rise to the Rontgen rays in the Rontgen lamp, I observed about half-a- 
dozen faint images. These became fewer in number as the length of the air- 
spark was decreased, and were probably due to discharges from the walls of 
the Rontgen lamp. 
