284 Mr. H. F. W. Burstall on the Measurement 



(iii.) From the nature of the thermometer chosen, the 

 leads must be well insulated from each other and from 

 the body of the instrument, the insulating material 

 being such as to stand high temperatures. 

 To fulfil all these conditions was by no means easy. In 

 order to satisfy the first I was forced to employ a naked wire 

 of small diameter, which obviously gives the minimum ther- 

 mal capacity per unit of surface. I was fully aware that the 

 whole of Callendar and Griffiths' s work had been done on wires 

 which were carefully shielded from the least contamination by 

 means of an outer envelope; hence it might have been expected 

 that a naked wire would prove unreliable, but I show later 

 that, provided suitable precautions are taken, naked wires give 

 satisfactory results, at any rate to the order of accuracy of my 

 experiments. 



The difficulties of obtaining the requisite mechanical 

 strength, and at the same time good insulation, were very 

 great; and nearly twelve months' work was expended 

 before a satisfactory solution of the problem was obtained. 

 The greater number of the earlier instruments were deficient 

 in mechanical strength. In most laboratory experiments this 

 is seldom of great importance ; but this weakness invariably 

 led to the thermometer being blown out of the engine after 

 a few explosions, and the results were disastrous to the 

 experiments. 



Description of the Thermometer. (Plate I. fig. 1.) 



The body of the thermometer consists of a seamless steel 

 tube 15 inches long and f inch outside diameter ; the outside 

 of the tube is screwed from end to end with a fine thread. 

 Working on this thread is a nut N, so arranged that the 

 thermometer-wire can be immersed to any required depth in 

 the gas-engine cylinder. For a distance of about one inch 

 from one end, A, the tube is threaded in the interior and 

 a small ring screwed in. Against this ring, which forms an 

 internal collar, rests a circular slate block (C) pierced with 

 four small holes through which the four leads pass. These 

 leads are of platinum, 0*03 inch diameter and 4 inches long. 

 About 1*5 inches from the end A small platinum collars are 

 gold-soldered on to the leads ; these prevent the leads from 

 being blown out of the tube, by bearing against the slate 

 block. In the stem of the thermometer the leads are of stout 

 copper soldered with brass to the platinum leads and separated 

 from each other by mica washers. After the leads have been 

 put in position, a packing of alternate layers of asbestos and 

 mica washers is placed on the slate block, as shown in the 



