THE RETURN CIRCUIT. 55 
stant in accordance with the new conditions, which are 
there actually found in practice. ‘The proper amount of 
increase in the constant is a little uncertain as is indicated 
by the table just given. For R' = .4 R however the con- 
stant is 14.4 so that we may rewrite the copper formula as 
follows: 
14.4CD 
=t 
In the vast majority of cases the constant will lie be- 
tween 14 and 15. The exact value to be assumed depends 
on the conditions as to track circuit and load in the par- 
ticular case considered, and can be judged approximately 
from the table. It may sometimes be desirable to make a 
few trial calculations with different constants in order to 
get a clear idea of the possible amount of copper. 
It is, of course, possible to determine a condition for 
minimum cost of the conducting system, taking account of 
the cost of copper, rails and bonding, but, generally speak- 
ing, the rail is fixed by purely mechanical considerations 
while there are, as has been shown, good reasons for making 
the track circuit thoroughly good. In applying the above 
formula, as we shall in the next chapter, it should be re- 
membered that in extensive systems the constant may have 
to be modified in passing from one locality to another, for 
the rail conditions will probably vary and the load condi- 
tions most assuredly will change, ; 
Co Wl —