Portable Capillary Electrometer. 401 



the rest of the instrument, for, unless the stand and the 

 support on which it rests are very rigid, the pressure 

 necessary to depress the spring produces sufficient movement 

 of the meniscus, by change in the inclination of the capillary 

 tube during the act of depression, to render the detection of 

 minute changes of surface-tension impossible. 



The mercury-key, represented in the second figure, is 

 free from these disadvantages. It consists, as shown, of a 

 U-tube closed at one end and communicating at the other 

 with a pneumatic-pressure ball and containing mercury in 

 the bend. Three platinum wires are fused into the tube and 

 connected as shown. It is obvious that the same change of 

 contacts is produced by squeezing the ball B, as by depressing 

 the lever S in the spring-key. The mercury in this key 

 takes the place of the lever in the spring-key, and the two 

 different contacts between it and E 3 and Jt*J 2 respectively are 

 here quite definite and practically independent of the amount 

 of pressure exerted upon the ball B. Further, the contacts 

 are not exposed directly to the laboratory atmosphere. The 

 thermoelectric effects are very small since the changing con- 

 tacts take place between platinum and mercury which are 

 almost identical thermoelectrically. The warmth communi- 

 cated to the key from the hand of the operator can be 

 neglected, and the heat produced by the compression of the 

 air in the key need only be very small. The key can be 

 fixed to the same stand as the rest of the apparatus, for even 

 if the pressure which changes the contacts is applied as 

 suddenly as possible the maximum vertical pressure upon the 

 stand is only a few grams, whereas in the case of the ordinary 

 spring-key it may be 500 grams or more. 



By adjusting the length of the mercury column in the key 

 so that it is only slightly less than the length of the U-tube 

 between the two extreme platinum wires, the change of 

 potential at P x from E 2 to E, and conversely, can be made 

 almost instantaneously. Hence measurements can be made 

 with the instrument even if the zero is altering fairly rapidly. 

 With a key of this kind it is also obvious that the observa- 

 tion of the meniscus through the microscope can receive a 

 very near approach to undivided attention. 



Commutators and keys which make a set of connexions in 

 a prescribed order can be constructed on the same principle 

 as the pneumatic key above described. The third figure (p. 402) 

 represents a combined commutator and electrometer key. 



The U-tube to the left is the key already described, and the 

 double U-tube to the right is the commutator. The positions 

 of the different wires are so arranged with respect to the 



