36 PROCEEDINGS OF THE AMERICAN ACADEMY. 



from B' without stopping the coustaut flow of steam through A, by means 

 of the siphou f, which is provided with a valve. The steam, after pass- 

 ing through the hot chamber, is led to the outer air by a jacketed pipe 

 h, descending* from the bottom of A. 



The connections of the thermal elements are led out of the sides of the 

 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 centimeters 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, p. The copper ends of 

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

 pair pitted against each other, may be (|uickly connected with a second 

 copper wire leading to 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 resistance, A', in the poten- 

 tiometer circuit can be so adjusted that every millimeter on the poten- 

 tiometer wire corresponds to an}' desired small ])Oteutial difference, such 

 as one microvolt or one tenth of a microvolt. Katlier tlian make this 

 adjustment many times a day to conform to the varying temperature of 

 the copper wire, however, we lind it better to determine the slight correc- 

 tions necessary to reduce the readings to absolute measure, by noting at 

 frequent intervals the indications of a standard thermal couple, the elec- 

 tromotive force of which is well known. The potentiometer wire, which 

 is 0.25 mm. in diameter, can be changed in a few seconds for new wire, 

 if the old should become dented or stretched. 



Into the vessel D about 100 kilograms of cracked ice can be put, and 

 this ice can be kept in constant motion over the smooth bottom by help 

 of the electric motor, M. 



Figure 7 shows a similar but smaller ai)paratus without its elaborate 

 system of inch thick asbestos jackets. Z? is a closed iron drum contain- 

 ing a rotary stirrer and rubber scraper turned by a motor. Through D 

 a large volume of water can be sent at a steady rate. The hot chamber 

 is the iron box, B, planed on its upper surface and communicating at the 

 bottom with a retort chamber, C, in whi(;h about 20 kilograms of mercury 

 can be kept boiling. The outlet at fallows the vapor to escape to the 

 tube^f, connecting with a large wrought iron chamber where it condenses 



* In the diagram, A is erroneously represented as ascendinfj, and as inserted in 

 the side of A. 



