500 
MR. E. H. GRIFFITHS OX THE VALUE OF 
Appendix I. 
Determination of the ‘'Null Point" and an alternative Method of calculating the 
Results. 
As stated in the introduction we append a full analysis of the “ null point,” i.e., the 
point at which the radiation is self-eliminated. 
Using a similar notation to that on p. 478, we have as the equation of condition 
= .w- 
where the temperatures are measured from that of the surrounding envelope. 
For the sake of simplicity we can assume that the values of a and M remain 
constant. 
Integrating and putting X = p/M, p = a/M, and determining the constant from 
the fact that when t = 0, 6 = — we obtain the equation 
- 
X 
If there had been no radiation, p = 0, and the equation of condition would have 
been dOldt = p. 
Integrating, and using the same constant as before 
0 = pt — 6 q .(3). 
If we find the points of intersection of (2) and (3) one point (— 6q, 0) is that at 
which the experiment commenced, the other (@, T) is the point on (2) at which the 
radiation is eliminated. 
It is more convenient, for experimental work, to obtain an expression involving T 
rather than © ; substituting therefore the value of 6 given by (3) in (2), we obtain 
\ a 
y -1- t'o _ 1 
p T 
= 0 . 
This equation can be solved for T when the values of X, p, $q are known. 
In order to obtain X, p, we can take the following observations: Commence an 
exjrej-iment at — 6q, note the time t when ^ = 0, and again note the time t.,, when 
^ ^ 0 - 
Then equation (2) gives the relations 
