Determining the Vapour Pressures of Solutions. 383 



made of low sensitiveness till the pressure is within a little of what 

 is desired, and then by closing the tap U its sensitiveness can be 

 increased tenfold. Thus the observer, with his eye on the scale of 

 the capillary and hand on the tap t 6 , can make a momentary con- 

 nection either with the exhaust pump or the external air, and so 

 regulate the pressure with great precision. 



§ 4. The Electrical Instruments. 



So much for the mechanical portion of the apparatus : it remains 

 to describe the electrical part. 



The outfit consists of two Callendar- Griffiths platinum thermome- 

 ters, a battery acting through a resistance of 50 ohms, a pair of 

 "equal arm" coils, a galvanometer, and a bridge whose 80 cm. of 

 wire balance the difference of resistance of the two thermometers, 

 thus indicating the difference of their temperatures. 



The method of compensating and connecting these thermometers 

 is fully described by Griffiths ;* the bridge, however, will require a 

 little comment. The wire was ordered to be 0'5 ohm to the metre, 

 to which value it approached very nearly. It was furnished with 

 the same platinum-silver wire as the standard instruments recently 

 supplied to Kew Observatory. The scale unit was approximately 

 2 cm. The bridge itself was furnished with Professor Callendar's 

 device for obviating the greater part of the thermo-electric effects by 

 connecting the galvanometer to a wire running parallel to the bridge 

 wire, and in all respects similar to it. These effects are further 

 removed by a Griffiths's thermo-electric key. 



All contacts were so designed as to fall within a very narrow 

 compass, where they can if necessary be enclosed in a box to prevent 

 injury. The equal arm coil was made of two pieces of wire having 

 (when wound) precisely the same resistance and temperature coeffi- 

 cient. They are intertwined in a core of paraffin so as to be at 

 equal temperatures. The actual temperatnre is then a matter of 

 indifference. 



The battery employed was a single dry cell. The galvanometer, 

 after repeated attempts, was made very sensitive. The manner of 

 reading its deflection differed somewhat from the ordinary one. The 

 method is similar to that of Poggendorff, except that the scale is in 

 the eye-piece of the telescope, which is focussed (by reflection 

 from the galvanometer mirror) on a signal. Instead, therefore, of 

 observing the movements of a scale relative to a fixed line in the 

 focal plane of the telescope, one observes the movements of a line 

 relative to a fixed scale in the focus of the telescope. In this there 

 is nothing new. But no one, so far as can be ascertained, has 

 * ' Phil. Mag.,' Jamiary, 1895. 



