288 MESSES. F. GOTCH AND V. HOBSLEY 



difference by means of a rheotome for only -foVo second. The ocular of the microscope 

 was fitted with one of ZEISS'S micrometers, the scale being worked across the field 

 by a screw mechanism. In the particular arrangement employed, the scale bore such 

 a relation to the actual dimensions of the capillary that one division corresponded 

 to 10 4 00 millimetre of the object. A difference of potential of roVo Daniell when 

 connected with the poles of the electrometer produced a movement of the mercury 

 amounting to yo millimetre, and this, when viewed by means of the eyepiece, 

 amounted to five divisions of the scale. 



The element of uncertainty in connection with the movement of the electrometer 

 under different conditions and the difficulty of obtaining reliable records of changes 

 differing from one another by constant but small amounts, induced us to abandon the 

 instrument for the quantitative observations which make up the bulk of the present 

 research. It was, however, often used as a means of ascertaining the existence and 

 character of electrical changes during the initial stages of a new series of experiments, 

 and proved in this respect a useful guide. 



The movements of the meniscus were in most cases observed by the eye, but in some 

 cases they were photographed upon a travelling sensitive plate, as in the experiments 

 described in our previous publication. 



(6.) The galvanometer was the instrument upon which we relied for results suscep- 

 tible of quantitative comparison. It was made by Messrs. ELLIOTT upon the lines of 

 THOMSON'S reflecting instrument, and had a resistance at 16 C. of 20,364 ohms. The 

 light magnetic system was effectually damped by being enclosed between two plates of 

 glass 2 mm. apart, the aluminium vane of the ordinary Thomson being dispensed with 

 to secure a decrease in the inertia of the system. 



The degree of sensibility employed was such that the needle and mirror gave a 

 deflection amounting by the reflecting method of observation used to 650 scale, when 

 the instrument was connected through a resistance of 10,000 ohms (external to its 

 own resistance) with a difference of potential amounting to '01 Daniell, and when 

 only Y^O of the current in the circuit was allowed by the shunt to traverse the instru- 

 ment. The small mass of the system enabled it to respond to currents of very short 

 duration, thus with an additional resistance of 10,000 ohms in the circuit, a difference 

 of '01 Daniell produced a deflection of 5 scale when connected by means of a rheotome 

 with the galvanometer for a period of only j^nfo second. 



The deflections of the magnetic system were measured by the usual reflecting 

 method ; a biconvex lens of 4 inches focal length was, however, introduced almost 

 3 centims. in front of the source of light between it and the concave mirror (of 

 40 inches radius) of the system. The screen behind which the light was placed was 

 pierced by a circular aperture bisected by a vertical cross wire, and the reflected 

 image on the scale was thus a large well-illuminated disc with a sharply-defined 

 vertical shadow dividing it. The position of the edge of the vertical .shadow on the 

 scale could be thus observed with great accuracy, since the moving illuminated disc 



