192 S. P. Langley—The Actinic Balance. 
nary form, but in its most elementary type, let us suppose that 
we have succeeded in rolling steel until its thickness is xf, to 
mm In this state 8,000 to 12,000 sheets laid one on the 
other will make but about one English inch. It may be easily 
supposed that it is no light task to procure such a sheet of steel 
= nb first instance. This, however, has been done success- 
ully. 
In order to fix our ideas let us now suppose two such pieces 
of steel, each rather less than } inch long and ¥, inch wide, to 
be stretched side by side and almost in juxtaposition, within a 
small cylinder open at one end, which can be directed to the 
source of radiant heat, while the two strips are made to form 
each a portion of a circuit leading from the battery to the differ- 
ential galvanometer. Since the change of resistance in iron 1s 
about 4; of one per cent for each degree Centigrade, as minute 
a change of temperature as is represented by a single degree 
will cause a difference of resistance in the strips of x45: us 
supposing the resistance of the exposed part to be $ that of the 
whole circuit, there will be a differential effect upon the galvan- 
ometer equal to nearly z;4;5 of the entire power of the battery, 
an enormous amount of force as compared with that represented 
In 
reality it is not so. The current received from the battery 
d 
eo 
