28008 Mr Whiddington, Note on the Absorption 
Note on the Absorption of Cathode Rays by Metallic Sheets. 
By R. Wuippineron, M.A., St John’s College. 
[Received 17 June 1913.] 
Last year I published an account of some experiments on the 
transmission of cathode rays through matter which showed that 
the law determining the diminution of velocity of cathode rays 
through matter was expressed by the relation 
= Ue S Oho B, 
where 
U= the velocity of a beam of rays incident on a sheet of 
material of thickness 2, . 
Vz= the greatest velocity in the emergent stream, 
and a =a constant depending on the material of the sheet. 
We are thus led to the conception of range of cathode rays, for 
if in the above expression we put v;=0 the value of »,*/a is the 
thickness of the material through which cathode rays of velocity », 
can just penetrate. 
In fact by using this expression we can easily determine the 
constant a by measuring the velocity v,, below which no appreci- 
able effect is produced on the emergent side of a sheet of thickness 
d; we can then substitute these values in d = 2,'/a. 
I have recently been making experiments with cathode rays of 
definite speeds in order to determine the number of the rays 
absorbed during their passage through metals. 
Lenard, Seitz and others many years ago showed that using 
rays of a more or less definite speed, the law of absorption was 
exponential, being expressed by the relation 
Lele, 
where 
I,=the cathode ray current incident on a metal sheet of 
thickness «, 
I =the corresponding emergent cathode ray current, 
and ) =the absorption coefficient, a constant for any definite 
speed but varying inversely as the fourth power of 
the speed of the incident cathode rays. 
I have found that in all cases I have examined this law is by 
no means universally applicable. In fact the experiments were 
originally commenced with the idea. of discovering whether 2X 
suffered an abrupt change when the material of the absorbing 
screen commenced to give out its fluorescent Réntgen radiation. 
So far, experiments have been carried out using screens of Au, 
Ag, Cu and Al. 
Sie 
