242 REPORT—1874. 
Report of the Committee, consisting of Prof.A.W.Wit11aMson, F.R.S., 
Prof. SirW. Tuomson, F.R.S., Prof. Clerk Maxwett, F.R.S., Prof. 
G. C. Foster, F.R.S., F. A. Anet, F.R.S., Prof. Finmmine Jenxin, 
F.R.S., C. W. Stemens, F.R.S., and Mr. R. Sante, appointed for 
the purpose of testing the new Pyrometer of Mr. Siemens. 
Four pyrometers, numbered by the makers 404, 411, 414, and 445 respec- 
tively, have been supplied for the purposes of the Committee by Dr. Siemens 
during the course of the experiments. In all the instruments the part by 
which indications of change of temperature are given is identical, and con- 
sists of a length of fine platinum wire doubled back on itself, and coiled upon 
a cylinder of refractory fire-clay. The ends of the coil are fastened to stout 
platinum wires of such a length that their further extremities never reach a 
very high temperature, and these in their turn are connected by copper wires 
with binding-screws on the outside of the case of the pyrometer. The copper 
wires are enclosed in a stout tube of wrought iron, about 3:5 centimetres in 
diameter and about 120 centimetres long, which projects from the furnace 
or other space whose temperature is required, and forms a handle and support 
for the whole instrument. The part to be inserted in the furnace (namely, 
the coil of platinum wire) is protected by a case or sheath, which is fastened 
by screws to one end of the iron tube. In pyrometer No. 404 (which, it is 
understood, was constructed according to the plan usually adopted by Messrs. 
Siemens Brothers previously to the experiments of the Committee) this sheath 
was made of a piece of wrought-iron tubing closed at one end, and the fire- 
clay cylinder supporting the platinum coil was inserted in it without any 
further protection, beyond a packing of asbestos, employed to prevent it 
shifting or being injured by shaking. Pyrometers Nos. 411 and 414, besides 
an outer sheath of wrought iron similar to that of 404, had a piece of stout 
platinum foil wrapped tightly round the fire-clay cylinder, but of course not 
touching the coil. In all other essential respects they were exactly like 
No. 404. In No. 445, instead of a wrought-iron sheath, a platinum tube 
closed at one end was used to enclose the pyrometric coil, so that in this in- 
strument, although it had the usual long stem of wrought iron, no part of 
the iron was ever exposed to a red heat ; otherwise, it was just like the others. 
In all cases the conducting-wires, where they pass up the stem of the instru- 
ment, are kept from contact with it or with each other by being inserted in 
clay tubes like tobacco-pipe stems, 
The indications depend on the changes which the electrical resistance of 
the platinum coil undergoes when its temperature is altered. In ordér to 
avoid the errors that might otherwise arise from the heating of the leading 
wires connecting the pyrometer with the measuring-apparatus, the undivided 
current of the testing-battery is conveyed by a wire, which passes down the 
stem of the instrument, and is denoted in the diagram by C, to the beginning 
of the pyrometer-coil, where it divides into two parts, one of which, after 
traversing the coil, is conveyed up the stem and back to the battery by a wire 
marked X, in the diagram, while the other part is conveyed by a precisely 
similar wire (X) to the standard against which the coil is to be measured. 
Thus, in the comparison, the resistance of the wire X, acts as an addition to 
that of the pyrometer-coil, and that of the wire X as an equal addition to 
the resistance of the standard. To insure that this compensation between 
the resistances of the leading wires is as accurate as possible, the three wires, 
