June 30, 1892] 



NATURE 



21 



fixed to their lower ends below the base plate of system A. 

 Thumbscrew nuts, /, are placed upon the upper ends of the 

 bolts after they pass through the holes in the top plate of 

 system A. 



When the instrument is set up ready for use, these thumb- 

 screws are turned up against fixed stops, g, so as to be well clear 

 of the top plate of system A ; but when the instrument is packed 

 for carriage they are screwed down against the plate until the 

 conical nuts mentioned above are drawn up into the conical 

 holes in the top and bottom plates of system A ; system B is thus 

 raised off the glass pillars, and the two systems are securely 

 locked together so as to prevent damage to the instrument. 



A dust-tight cylindrical metal case, h, which can be easily 

 taken off for inspection, covers the two systems, and fits on to a 

 flange on system A. The whole instrument rests on three 

 vulcanite legs attached to the brass plate on system A ; and two 

 terminals are provided, one, i, on the base of system A, and 

 the other,/, on the end of one of the corner bolts of system B. 



The air leyden which has been thus described is used as a 

 standard of electrostatic capacity. In the instrument actually 

 exhibited to the Society there are twenty-two plates of the 

 system B, twenty three of the system A, and therefore forty- 

 four octagonal air spaces between the two sets of plates. The 

 thickness of each of these air spaces is approximately 0*301 of a 

 centimetre. The side of each square is io'i3 cm., and therefore 

 the area of each octagonal air space is 85*1 sq. cm. The 

 capacity of the whole leyden is therefore approximately 

 44 X 85"l/ (4ir X •287), or 1038 cm. in electrostatic measure. This 

 is only an approximate estimate, founded on a not minutely 

 accurate measurement of dimensions, and not corrected for the 

 addition of capacity, due to the edges and projecting angles of 

 the squares and the metal cover. I hope to have the capacity 

 determined with great accuracy by comparison with Mr. Glaze- 

 brook's standards in Cambridge. 



To explain its use in connection with an idiostatic electro- 

 meter for the direct measurement of the capacity of any insulated 

 conductor, I shall suppose, for example, this insulated con- 

 ductor to be the insulated wire of a short length of submarine 

 cable core, or of telephone, or telegraph, or electric light cable, 

 sunk under water, except a projecting portion to allow external 

 connection to be made with the insulated wire. 



The electrometer which I find most convenient is my " multi- 

 cellular voltmeter," rendered practically dead-beat by a vane 

 under oil hung on the lower end of the long stem carrying the 

 electric "needles" (or movable plates). In the multicellular 

 voltmeter used in the experimental illustration before the Royal 

 Society, the index shows its readings on a vertical cylindric 

 surface, which for electric light stations is more convenient than 

 the horizontal scale of the multicellular voltmeters hitherto in 

 use ; but for the measurement of electrostatic capacity the older 

 horizontal scale instrument is as convenient as the new form. 



To give a convenient primary electrification for the measure- 

 ment, a voltaic battery, vv, of about 150 or 200 elements, of 

 each of which the liquid is a drop of water held up by the 

 capillary attraction between a zinc and copper plate about 

 I mm. asunder. An ordinary electric machine, or even a stick 

 of rubbed sealing-wax, may, however, be used, but not with the 

 same facility for giving the amount of electrification desired as 

 the voltaic battery. 



One end of the voltaic battery is kept joined metallically to a 

 wire, W, dipping in the water in which the cable is submerged, 

 and with the case C of the multicellular, and with the case and 

 plates A of the Leyden, and with a fixed stud, S, forming part 

 of the operating key to be described later. The other end of the 

 voltaic battery is connected to a flexible insulated wire, FFF, 

 used for giving the primary electrification to the insulated wire 

 J of the cable, and the insulated cells, II, of the multicellular 

 kept metallically connected with it. The insulated plates B of 

 the leyden are connected to a spring, KL, of the operating key 

 referred to above, which, when left to itself, presses down on 

 the metal stud S, and which is very perfectly insulated when 

 lifted from contact with S by a finger applied to the insulating 

 handle H. A second well insulated stud, S', is kept in metallic 

 connection with J and I (the insulated wire of the cable and the 

 insulated cells of the multicellular). 



To make a measurement, the flexible wire F is brought by 

 hand to touch momentarily on a wire connected with the stud 

 S', and immediately after that a reading of the electrometer is 

 taken and watched for a minute or two to test either that there 

 is no sensible loss by imperfect insulation of the cable and the 



NO. 1183, VOL. 46] 



insulated cells of the multicellular, or that the loss is not suffi- 

 ciently rapid to vitiate the measurement. When the operator 

 is satisfied with this, he records his reading of the electrometer, 

 presses up the handle H of the key, and so disconnects the 

 plates B of the leyden from S and A, and connects them with 

 S', J, I. Fifteen or twenty seconds of time suffices to take the 

 thus diminished reading of the multicellular, and the measure- 

 ment is complete. • • 



The capacity of the cable is then found by the analogy :— As 

 the second reading of the electrometer is to the excess of the 



first above the second, so is the capacity of the leyden to the 

 capacity of the cable. 



A small correction is readily made with sufficient accuracy for 

 the varying capacity of the electrometer, according to the 

 different positions of the movable plates, corresponding to the 

 different readings, by aid of a table of corrections determined 

 by special measurements for capacity for the purpose on the 

 multicellular. 



UNIVERSITY AND EDUCATIONAL 

 INTELLIGENCE. 



Cambridge. — Prof. Liveing announces a course of lectures 

 in general chemistry, to be given during the Long Vacation by 

 Mr. Fenton, beginning on July 7. Mr. Fenton will also give a 

 series of demonstrations on the chemistry of photography. 



At the Congregation on June 16, seven graduates in arts were 

 admitted to the degree of Doctor in Medicine, and thirty-one to 

 the degrees of Bachelor of Medicine and Bachelor of Surgery. 

 These are the largest numbers hitherto admitted at one time. 



Sir R. S. Ball, Lowndean Professor of Geometry and Astro- 

 nomy, has been elected to a Professorial Fellowship at King's 

 College. 



At Christ's College the following awards have been made to 

 students of natural science :— Scholarships : E. K. Jones (;^5o), 

 G. A. Anden (;^30), J. M. Woolley (;^3o), C. F. G. Master- 

 man (;^5o), H. Pentecost (;^So), A. M. Hale (;^3o). Exhibi- 

 tion : A. M. Barraclough (;^3o). At Emmanuel College : — 

 Scholarship : A. Eichholz (;^8o). Exhibition : J. C. Muir 

 (;^3o). 



At the annual election of scholars in St. John's College, the 

 following awards in Natural Science have been made : — Founda- 

 tion Scholarships : W. L. Brown, T. L. Jackson, W. 

 McDougall, S. S. F. Blackman. Exhibitions in Augmentation 

 of Scholarships : Villy, Whipple (First Class Nat. Sci. Tripos, 

 Part II.). Hughes Prize (highest in third year) : Villy. Her- 

 schel Prize in Astronomy : Pocklington. Hutchinson Student- 

 ship for Research in Zoology : E. W. MacBride. 



