A. L. Day and J. K. Clement — Gas Thermometer. 411 



magnitude in an absolute scale attract altogether inadequate 

 attention. This is obviously no aspersion upon the beautiful 

 work of Jaquerod and Perrot, or of any other investigator, but 

 it may be the explanation of the uncertainty in existing high 

 temperature measurements. Jaquerod and Perrot, for example, 

 measured the melting point of gold with the gas thermometer, 

 using five different gases successively in the same (fused silica) 

 bulb, with a maximum variation of only '4°, and yet in its 

 absolute value the determination may easily be 5° or more in 

 error. In fact, in one of their determinations in which a por- 

 celain bulb was substituted for silica, a difference of 4° 

 appeared. The observation was dropped, but it serves to direct 

 attention sharply to a possible uncertainty of several degrees 

 arising from the corrections for the distribution of temperature 

 along the bulb and the unheated space, and for the expansion 

 coefficient of the bulb itself. 



Somewhat more in detail, the apparatus may be described 

 as follows : 



The Furnace. — The furnace consists of a wrought iron tube 

 of about 25 cm inside diameter, carrying a cast iron pipe flange 

 at each end. To these flanges cast iron covers were fitted by 

 grinding to a gas-tight joint. In position this bomb is vertical, 

 and the lower cover is permanently secured in place with bolts. 

 The furnace tube is made from a magnesite mixture* about 

 36 cm long and 6 cm inside diameter within which the furnace 

 coil is wound. This scheme of winding the heating coil on the 

 inside of a refractory tube is very successful in its operation 

 and is not difficult. With a pure platinum coil (melting point 

 about 1750°) a furnace temperature of 1600° can be reached 

 without danger to the coil and maintained for some time if 

 desired. There is considerable loss of platinum through 

 sublimation in maintaining a resistance furnace at this tempera- 

 ture, so that it is necessary to use a wire of considerable size if 

 it is required to maintain so high a temperature for long periods 

 of time. The gain over the same coil wound on the outside of 

 a thin porcelain tube is about 200° (1600° instead of 1400°) 

 for the same current and conditions of insulation. The method 

 of winding is simple. A series of five wooden wedges is 

 grouped together so as to collapse when the center one is 

 removed. When grouped and fastened together the outside 

 surface is turned down to a cylinder of exactly the size which 

 the finished coil is to have. This multiple wedge then serves 

 as a collapsible arbor and the coil is wound upon it with any 

 desired arrangement of turns. A piece of paper or thin card- 

 board between the wire and the arbor sometimes facilitates the 

 removal of the arbor after completion. The arbor with the 



* Harbison-Walker Eefractories Company, Pittsburg, Pennsylvania. 



