Constant Pressures and Temperatures. 415 



screw K is connected with a small piece of steel tube inter- 

 posed between the end of AB and the india-rubber tube. 

 This third connexion with the mercury contained in the mano- 

 meter may of course be made by means of a third (insulated) 

 rod passing down to the bottom of the wider part of A B or F. 



To fill and adjust the manometer, the tube F is raised to the 

 same level as A, and mercury is poured in until it just reaches 

 the wide cylindrical portions of A and F; the rods E 1? E 2 are 

 then moved until the point of E x just touches the surface of 

 the mercury, while that of E 2 is a fraction of a millimetre 

 above it. 



Suppose now that F be lowered n millims. and the air- 

 pump be set to work to pump air out at D ; further, that K 

 be connected with one pole P of a battery, and the rods 

 E l7 E 2 with the other pole N ; then it is evident that as long 

 as the pressure in A, and consequently in X, is greater than 

 H— n (where H= the barometric pressure), the current will 

 pass through the circuit P K E 2 N, whereas when the pressure 

 is less than H— n the current will pass along PKE^. 

 Similarly, if F be raised n millims. above A and the pump be 

 made to compress air into A, as long as the pressure is less 

 than H + ?i, the current will pass along PKE X N; but when 

 the pressure becomes greater than H + n, the current will pass 

 along P K E 2 N. In order to economize space, the tube A B 

 is widened out at B ; and when it is intended to obtain a 

 pressure greater than H, the rod E L is replaced by a long one 

 reaching down to B ; this, of course, is equivalent to lowering 

 A or raising F nearly the whole length of the scale S S. In 

 the further description I shall only consider the case in which 

 F is lower than A — that is, when a pressure less than H is 

 required; the alteration needed for higher pressures will readily 

 suggest itself. 



The apparatus depicted in fig. 3 consists of a brass tap T and 

 an electromagnetic clutch to work the tap automatically. The 

 tap, which is shown in section in fig. 4, is placed between the 

 tube D of the manometer (fig. 2) and the air-pump, L x being 

 connected with D and L 2 with the pump. From the figure it 

 is readily seen that when the tap is in the position drawn, and 

 the lever fitted to the head of the key lies in the direction a b, 

 air will be admitted into the manometer ; if, on the other hand, 

 the lever and key occupy the position a' ¥, the pump will with- 

 draw air from the manometer. The object of the clutch, there- 

 fore, is to place the tap in the first position when the pressure is 

 too small, and in the second when it is too great. The lever a b 

 (fig. 3) terminates in two arcs which are grooved to hold a cord; 

 these arcs are furnished with set screws Si, S 2 , by means of which 



212 



