490 Prof. A. Schuster on the Scale- Value of 



The Apparatus used in the Comparison of Thermometers. 



The comparison of the thermometers was carried out in a 

 bath made of sheet-iron having a length of 114 centim. and 

 width and depth of 20 centim. This bath was placed for pro- 

 tection inside a wooden box, on the bottom of which it rested 

 upon two ribs covered with guttapercha, so that the inner 

 vessel was practically insulated thermally. The wooden box K 

 is shown in PL VI. fig. 4, placed on two stools and in front of 

 a table T. A frame FF fitted into the bottom of the bath and 

 carried 38 turns of No. 21 nickel wire, the ends of the wire 

 being brought to binding-screws placed at two of the corners 

 of the outer box. With a suitable electric current passing- 

 through the wire, the temperature of the water in the bath 

 could either be kept constant or increasing at a desired rate 

 within a range from 1° to 15° above the temperature of the 

 j*oom. The whole of the interior of the bath, also the frame, 

 and wire were coated with white paint. A tank of water of 

 this kind containing over 40 litres cannot with any reasonable 

 amount of stirring be kept at a sufficiently uniform tempera- 

 ture. The bulbs of the thermometers were therefore placed 

 into a small copper box B, within which the stirring was 

 much more efficient. The box was 15 centim. in breadth 

 and 10 centim. deep, and was rigidly suspended from a 

 wooden cross-bar resting on the side of the case. Vertically 

 down the centre of the box passed a spindle carrying a double 

 3-bladed screw-paddle ; one of these paddles was fixed just 

 below the bottom of the box, and the other just inside the 

 box, as shown in the figure. There was a further paddle at 

 the other end of the bath, the power being supplied by two 

 Cuttriss motors. The stirring was sufficient to secure a very 

 approximately uniform temperature all over the bath. The 

 box B sheltered the thermometers from outside radiation, 

 and protected them against jets of hot water coming from the 

 heated water, the paddle below the box being specially intended 

 to prevent irregular heating of the box. The water inside B 

 was thoroughly stirred, a mere rotation of the water being 

 prevented by oblique diaphragms fixed to the sides. One 

 further precaution was found advisable. Owing to evapora- 

 tion and radiation the water lost heat at its upper surface, and 

 the thermometers had to be protected against an inflow of cold 

 water through the opening through which the spindle passed. 

 This was done by a horizontal diskH fixed to the spindle, and 

 by covering the whole box as far as possible with asbestos. 

 The thermometer bulbs passed through -a window W 3'3 

 centim. wide, cut into one side of B, and a sliding shutter of 



