produced by Rontgen Rays in Lead and Zinc. 435 
fraction of the energy in the case of the zinc might escape in 
this form. It appeared from these that the total intensity of 
the secondary rays escaping from both surfaces of the zinc 
strip (as measured by the ionization produced) was less than 
1/15 of the primary rays absorbed. 
The energy measurements were again taken up, and a 
change was made in the method which would render it 
independent of the rate of loss of heat from the metals. The 
strips of lead and zinc were held at the ends by massive brass 
clamps, connected to binding screws outside the case by means 
of rods insulated from the case. In this way a known current 
of electricity could be sent through either strip ; the resistances 
of the strips were measured, and thus a known quantity of 
the energy could be developed in either strip and the 
corresponding deflexion of the radiometer obtained. The 
loss of heat through the electrodes was so rapid, however 
(especially in the case of the zinc), that measurable deflexions 
could not be got with the Rontgen rays. I accordingly 
substituted for each strip a five-barred grid, carefully cut 
from the same materials, each bar being one millimetre wide ; 
one electrode-clamp held the beginning of the first bar, 
the other the end of the fifth bar, so that the current passed 
through the five bars in series. This served the purpose 
although the deflexions produced by the rays falling on the 
zinc were still too small for very accurate measurement. 
The resistance of the zinc grid was 6*5 X 10 -5 ohm and of 
the lead 64 X 10 — 5 ohm ; currents of 10 to 12 milliamperes 
were used in the lead and from 35 to 50 milliamperes in the 
zinc. The deflexions produced by either grid were found to 
be proportional to the quantity of heat developed in it, but 
the sensitiveness of the two grids was very different. Thus 
in one experiment which may serve as an example of many 
which were made, the energy necessary to produce a deflexion 
of 1 cm. was : — with the zinc grid, 26*8 ergs per second ; 
with the lead grid, 7*82 ergs per second. When the 
Rontgen rays fell upon the zinc, the deflexion of the 
radiometer was 2' 6 cm. ; when on the lead, 10'8 cm. These 
measurements give 70 ergs per second in the zinc and 84 
ergs per second in the lead ; the ratio of the two is 1*2. 
This is a much smaller difference than was obtained in the 
Cambridge experiments ; and, what is more significant, 
any errors (due, for example, to a gain of heat by both strips 
from other portions of the apparatus struck by the rays, or 
to imperfect screening of one strip) would favour the lead 
on account of its greater sensitiveness. !So that the difference 
would be less than 20 per cent, rather than more. 
