402 



Mr. S. W. J. Smith 



on a 



Fig-. 8. 



levels of* the mercury, that it requires greater pressure to 

 change the contacts in the double U-tube, than in the tube to 

 the left. The first effect of 

 pressure is to throw the electro- 

 meter into the potentiometer 

 or other circuit, connecting P x 

 with E x and P 2 with E 2 . On 

 increasing the pressure, the con- 

 nexions in the double U-tube 

 are reversed and P L is con- 

 nected with E 2 and P 3 with 

 E,. Hence, when the contacts 

 change in the commutator, we 

 oet a motion of the electrometer 

 meniscus corresponding to ap- 

 proximately twice the difference 

 of potential between E T and 

 E 2 , and so, in this way, the 

 sensitiveness of the instrument 

 is doubled. 



While it is clear that to get 

 the minimum thermoelectric 

 effect in mercury-platinum keys, 

 it would be necessary to use a 

 method of chanoino- the con- 

 tacts in which compression of 

 air in the key does not take place, yet the thermoelectric 

 electromotive forces which occur in the pneumatic keys 

 described are much too small to produce an observable effect 

 upon the readings of the most sensitive capillary electro- 

 meter*. 



The sensitiveness of the electrometer, using the simple key 

 first described, is such that when the diameter of the wide 

 tubes is about 1 cm. and the diameter of the capillary is 

 about 1 mm., a movement of the meniscus perceptible with 

 certainty in a microscope magnifying 100 times is produced 

 by a potential-difference equal to "0001 volt. The actual 

 extent of the movement is somewhat variable, and amounts 

 usually to about *01 mm. The follov/ing numbers, obtained 

 with apparatus previously described (Phil. Trans. A. 189y, 

 vol. cxciii. p. 63), show the effect of comparatively large 



* Since this paper was read I have found that mercury keys (similar 

 in principle to those referred to above) in which the thermoelectric 

 effects are reduced to a minimum are described by Kamerlingh Onnes, 

 Ley den Communications, No. 27, p. 31, 1896. 



