S. P. Langley—The Actinic Balance. 191 
any given centimeter of its length will not exceed one ten- 
thousandth of an ohm, and accordingly, if from any source of 
radiant heat we let fall on the wire a ray which we will sup- 
pose to be one square centimeter in section, if it altered the 
resistance of the wire by so much as one one-hundredth part 
where it fell, we should have but one one-millionth of an ohm 
to produce the requisite change in the recording instrument. 
Evidently we must form the particular minute portion of the 
circuit on which the ray is to fall of some conductor which has 
a very high resistance indeed, as compared with the average 
resistance of the wires. If for instance we introduce a bit of 
gold-foil having a resistance of one ohm to the single centi- 
meter so that it shall form a virtual portion of one of the 
wires, and if we let the ray fall on this, we now produce a 
change equal to the one one-hundredth part of one ohm, whic 
is ten thousand times the effect produced before by the same 
cause. Similar considerations show us that the cylindrical 
form of the wire is a bad one, and that the metal used for the 
a exposed to radiant heat should be laminated, when it may 
€ made to present a much greater surface to the source of radi- 
ant heat, with precisely the same conductibility. It is clear 
then that the following conditions should unite; great electric 
resistance ; considerable change in resistance for a given change 
in heat, and a form which enables it to take up and part with 
heat very rapidly. 
ro this we may add among the minor conditions that it is 
desirable that the exposed portion should be of a metal readily 
reduced to thin lamine, that it should be non-oxidizable, since 
It 18 to be in an excessively thin strip, and that it should also 
have sufficient rigidity to preserve its form. The whole of 
these conditions can rarely or never be found in the same sub- 
stance. We must select our metal with a view to those con- 
ditions which are of most importance. Experiments were 
ona greet variety of metals. They finally conducted me to the 
Iron (or steel), platinum and palladium as the most avail- 
able ones. Gold in the form of foil is unsuitable on account of 
minute rents made by the blows of the hammer. Metals on 
glass do not work as well on account of the heat taken up by 
the glass; but to attempt to narrate all the trials made would be 
useless. To comprehend the apparatus used, not in its ordi- 
