22 Prof. B. Osgood Peirce on the 



A represents the hot chamber, weighing about two hundred 

 kilograms, which rests in a thick jacket on a heavy table or 

 stand made to hold it. A is connected directly with one (B) 

 of two stout-walled copper boilers, B and B', each of which 

 holds about 40 litres of water. A light cup-shaped weight, 

 inverted and laid on a large tube with squared end which 

 projects above the top of the boiler, acts as a sensitive safety- 

 valve and prevents any appreciable rise in temperature within 

 the boiler. B can be refilled when necessary with boiling 

 water from B / without stopping the constant flow of steam 

 through A, by means of the siphon /which is provided with 

 a valve. The steam, after passing through the hot chamber, 

 is led to the outer air by a jacketed pipe h descending from 

 the bottom of A. 



The connexions of the thermal elements are led out of the 

 sides of tbe prism shut in by A and Z, and are held between slabs 

 of wood, which act as a sort of guard-ring jacket to the prism 

 for about 40 centiin. before they emerge. The platinoid or 

 german-silver leads of these thermal junctions within the prism 

 are soldered together, and to a copper wire leading to the 

 (copper) wire of a potentiometer. The copper ends of the 

 couples lead to a mercury-switch by which any one of them, or 

 any pair pitted against other, may be quickly connected with 

 the potentiometer. On its way from the switch to the cold 

 junctions in C through the potentiometer-wire the current 

 encounters only copper. By means of a somewhat elaborate 

 standard potentiometer, not shown in the diagram, the resist- 

 ance, R, in the potentiometer- circuit can be so adjusted that 

 every millimetre on the potentiometer-wire corresponds to any 

 desired small potential-difference, such as one microvolt or one- 

 tenth of a microvolt. Bather than make this adjustment many 

 times a day to conform to the varying temperature of the copper 

 wire, however, we found it better to determine the slight cor- 

 rections necessary to reduce the readings to absolute measure, 

 by noting at frequent intervals the indications of a standard 

 thermal couple, the electromotive force of which is well known. 

 The potentiometer-wire, which is 0'25 miliim. in diameter, can 

 be changed in a few seconds for a new wire, if the old should 

 become dented or stretched. An iron casting, Z, seen in plan 

 in fig. 3 and in elevation in fig. 2, accurately planed below 

 and turned true above, is the bottom of the box. Between 

 this casting (which can be bolted to A) and A is held the prism 

 to be experimented on. While Z was in the lathe a small 

 hole, H, about 3 miliim. in diameter and 4 miliim. deep, was 

 drilled exactly in the centre of its upper face. Subsequently 



