WILLIAM .s//-:.i//-:.v.v, I-.R.S. 165 



of iron or platinum, and is fitted into the long tube, CC, 

 which is of wrought iron, and serves as a handle. When the lower 

 end of the casing is of iron, there is a platinum shield to protect 

 the coil on the pipe-clay cylinder. The purpose of the platinum 

 casing is to shield the resistance wire against hot gases, and against 

 a ccii lent. At the points, A A, fig. 7, the thick platinum wires are 

 joined to copper connections, over which pieces of ordinary clay 

 tobacco-pipe tube are drawn, terminating in binding screws 

 fitted to a block of pipe-clay, closing the end of the tube. A 

 third binding screw is provided, which is likewise connected 

 with one of the two copper connecting wires, serving to eliminate 

 disturbing resistances in the leading wires, as will be explained 

 in the third pirt of this paper. 



If temperatures not exceeding a bright red heat are to be 

 measured, the platinum protecting tube may be dispensed with 

 and iron or copper substituted.* 



INSULATION OF PIPE-CLAY CYLINDER. The pipe-clay tube, 

 upon which the platinum wire is wound, is, when cold, highly 

 insulating ; when heated, its conducting power increases, though 

 not to such an extent as to occasion any perceptible error. 



In order to investigate the extent of this increasing conductivity, 

 I coiled a length of platinum wire round a pipe-clay pyrometer 

 cylinder in the ordinary way between the leading wires, and then 

 cut the wire at the bottom, so that the current passing between 

 the leading wires would have to traverse the body of the pipe-clay, 



* In experimenting with pyrometers with platinum casings, no appreciable de- 

 terioration of the platinum wire or change in its conductivity at Centigrade 

 has been observed, beyond what is due to the complete annealing of the wire in 

 the first instance. With a view, however, of saving expense, the protecting tube 

 of subsequent instruments was made of wrought iron ; and an instrument of this 

 construction was submitted for trial to a committee appointed by the Uritish 

 Association in 1872-73. To my surprise it was found that each time, after the 

 coil had been exposed to intense heat, the platinum resistance at standard tem- 

 perature was permanently increased ; and, on examining the wire, it was found to 

 present a rough surface, and had become brittle. Prof. A. W. Williamson, the 

 Chairman of this committee, suggested that this change might be owing to the 

 reducing atmosphere produced by the highly heated iron casing, which would 

 cause the platinum to combine with a trace of reduced silicon, taken from the 

 pipe-clay cylinder in contact with the same. An analysis by Prof. Williamson 

 of the altered wire confirmed this view, and proved beyond doubt the necessity of 

 an oxidizing or neutral atmosphere within the protecting chamber. This condition 

 will be best obtained in making the protecting casing of platinum ; but for ordi- 

 nary purposes an iron casing well enamelled on the inner surface, or containing a 

 lining of porcelain, will answer equally well. 



