PYROMETER 



681 



of clay is sot ; the latter contracts by the heat, and the convergence is therefore in- 

 creased. Its amount being measured, the heat to which the cylinder has been exposed 

 can bo calculated. 



A good pyrometer is an instrument much wanted. Messrs. Siemens Brothers have 

 devised a pyrometer which essentially consists of a length of fine platinum wire 

 doubled back upon itself, and coiled upon a cylinder of refractory clay. The ends of 

 the coil are fastened to stout platinum wires of such a length that their farther extre- 

 mities never reach a very high temperature, and these in their turn are connected 

 by copper wires with binding-screws on the outside case of the pyrometer. The 

 copper wires are iuclosed in a stout tube of wrought iron, about 3'5 centimeters in 

 diameters and 120 centimeters long, which projects from the furnace or other space 

 whose temperature is required, and forms a handle and support for the whole instru- 

 ment. 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. 



The indications depend on the changes which the electrical resistance of the platinum 

 coil undergoes when its temperature is altered. Fig. 1699 will fully explain tlm 



1699 



X' 



whole arrangement. In order to avoid the errors which might otherwise arise from 

 the heating of the leading wires connecting the pyrometer with the measuring appa- 

 ratus, the undivided current of the testing battery is conveyed by a wire, which passes 

 down the stem of the instrument, and is denoted by c in the figure, 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 the wire marked x, 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-wire x acts 

 as an addition to that of the pyrometer-coil, and that of the wire x' as an equal addi- 

 tion to the resistance of the standard. In the figure, G represents the resistance coils ; 

 E the platinum-silver resistance coils ; b the testing battery ; g the galvanometer ; and 

 k the key. 



A committee of the British Association was appointed to report on the action of this 

 electrical pyrometer. Some of their results will be found in the ' Reports of the British 

 Association for 1872 and 1873.' 



An Acoustical Pyrometer. At the American National Academy of Science, at Cam- 

 bridge, U.S.A., an interesting paper has been read by Dr. A. M. Mayer, of Hoboken, 

 on an acoustical pyrometer. He commenced by giving a description of his method of 

 measuring the surfaces of sound-waves. An open organ-pipe, provided with a Konig's 

 nodal capsule, is connected with a small gas-jet placed in front of a revolving cubical 

 mirror. Alongside of this jet is placed another of Konig's capsules, also furnished 

 with a gas-jet, and attached to a heavy rubber tubing, some meters in length; to the 

 other end of this tube one of Helmholtz's resonators is attached by means of its beak. 

 The resonator and organ-pipe must both be of the same note. On sounding the organ- 

 pipe the gas-jet attached to its capsule is alternately increased and diminished, and 



