c 



TRANSACTIONS OF SECTION A. 497 



perimented upon, and no other sutstance in contact with it, in a liei-metically sealed 

 wlass vessel are advantanres in the method now described. It is also interesting to 

 remark that in this method we have continuity through the fluid itself all at one 

 equal pressure exceeding the critical pressure, but at different temperatures in 

 different parts, varying continuously from something above the critical tempei-ature 

 at the top of the tube to a temperature below the critical temperature in the lower 

 part of the tube. 



The pressure may actually be measured by a proper appliance on the outside of 

 the lower part of the tube to measure its augmentation of volume under applied 

 pressure. If this is to be done, the lower thermal vessel must be applied, not 

 round the bottom of the tube, but round the middle poi'tion of it, leaving, as already 

 described, 10 or 20 cms. above for observation of the surface of separation between 

 liquid and vapour, and leaving at the bottom of the tube 20 or 30 cms. for the 

 pressure-measuring appliance. 



This appliance would be on the same general principle as that adopted by Pro- 

 fessor Tait in his tests of the Challcmjer thermometers under great pressure (' Proceed- 

 ings Royal Soc. Edin.,' 1830) ; a principle which I have myself used in a form of 

 depth-gauge for deep-sea soundings ; in which the pressure is measured, not by the 

 compression of air, but bj' the flexure or other strain produced iu brass or glass or 

 other elastic solid. 



6. On tlie possibility of originating Wave-cHsl iirhances in the Ether hy 

 Electro-magnetic Forces. By Qc. F. Fitzgerald. 



7. On the Ntiviber of Electrostatic Units in the Electro-magnetic Unit. 

 By R. Shida, M.E., Imperial College of Engineering, Tokio, Japan. 



The obiect of this paper is to explain measurements made during the month of 

 July last for an evaluation of ' c,' the number of electrostatic units in the electro- 

 ma"-netic unit — a question which has much engaged the attention of the British 

 Association. We can evaluate ' i' ' by determining the electrostatic and also 

 tl\e electro-magnetic measure of anyone of the following terms: Electro-motive 

 Force, Resistance, Cm-rent, Quantitj^ and Capacity. It is tlie first of these terms 

 that I measured in the two systems of units, and the E. M. F. was that of Sir 

 AVm. Thomson's gravity Daniell, which is very constant. The question divides itself 

 into two parts. 



(A). Absolute electrostatic measurement of the E. M. F. 



This measurement was made by means of Sir AVm. Thomson's Absolute 

 Electrometer, the most perfect instrument of the land hitherto invented. As the 

 description and principle of this instrument will be found fully given in Sir 

 Wm. Thomson's ' Electrostatics and Magnetism,' I need not enter into these 

 explanations. I may mention, however, that the instrument, perfect as it is, will 

 not give accurate results unless considerable care be taken in using it. 



In measuring an E. M. F. by this instrument, it is important tliat the potential 

 of the jar or the guard ring or disc should be kept constant during the experiment. 

 It was observed, however, tliat the jar was losing its charge, though very slowly, 

 on account of the pieces of ebonite in the replenisher insulating Imperfectly. Of 

 course I could keep the potential of the jar the .same during the experiment by 

 means of the replenisher ; but I found it very diflicult to work the replenisher, and 

 to take at the same time accurate readings. For this reason I thought it better, 

 when the experiment is done by one experimenter, (or even when, I venture to think, 

 there are more experimenters than one) to proceed in the following manner. First, 

 connect one pole, say zinc, to the continuous plate, and the other pole to the outside 

 of the jar, and take a reading ; then reverse the poles and take another reading. 

 Repeat the same operation — that is to say, take a great number of readings by 

 successive reversals. If the experimenter be well practised, the time each reading 

 will take him will be very nearly the same. Let D,, U.„ D,, kc, be the readings 

 1880. K K 



I 



