858 



SCIENCE. 



[N. S. Vol. VIII. No. 207. 



face fauna during the second week of June, 

 and medus£e of Eucopewere found at vari- 

 ous times during the summer. On July 

 28 an undetermined species of Hydrome- 

 dusa was so abundant at Menimsha that a 

 tumbler simply dipped into the ocean would 

 be more than half tilled with them. 



Dr. Murbach has found Corynitis breeding 

 during July and the earljr part of August, 

 Podocoryne and Hypolytus during August. 



Ctenophores, frequent during the early 

 part of summer, literally swarm during the 

 latter part of August. Mneniojms is the 

 most abundant species. 



H. C. BuMPUs. 



AN AMPERE BALAAX'E* 



The Eeport of the Committee on Elec- 

 trical Standards for 1897 ended with the fol- 

 lowing paragraph : "It thus appears to be a 

 matter of urgent importance that a redeter- 

 mination of the electro-chemical equivalent 

 of silver should be made and that the gen- 

 eral question of the absolute measurement 

 of electric currents should be investigated 

 * * * " This work we were asked by the 

 Committee to carry out, and a grant of £75 

 was voted in its aid. We were thus led to 

 examine into the methods which had been 

 employed by Lord Eayleigh, Professor Mas- 

 cart and others, for determining the abso- 

 lute value of a current, as well as to con- 

 sider some other methods which have not, 

 as far as we know, been hitherto used. 



After much consideration we decided to 

 adopt a form of apparatus which, while 

 generally resembling the type employed by 

 some previous experimenters, possessed cer- 

 tain important differences ; and, before ex- 

 pending any part of the grant of £75, to 

 construct, without expense to the British 

 Association, the following preliminary Am- 

 pere Balance. 



On a vertical cylinder about 17 inches 

 high and 6.S inches in diameter we wound 



*Eead before the British Association. 



two coils, about 6 inches in height, separa- 

 ted by an axial distance of 5 inches. The 

 coils consisted each of a single layer of about 

 170 convolutions of wire and were wound 

 in opposite directions. From the beam of 

 a balance there was suspended, inside this 

 cylinder, a light bobbin about 4 inches in 

 diameter, on which was wound a coil about 

 10 inches long, consisting oi & single layer of 

 360 convolutions, and the whole apparatus 

 was so adjusted that when the beam of the 

 balance was horizontal the inner and outer 

 coils were coaxial and the top and bottom 

 of the inner suspended coil were respec- 

 tively in the mean planes of the outer sta- 

 tionary coils. 



This arrangement was adopted because 

 with coils consisting of only one layer the 

 geometrical dimensions could be accurately 

 determined, and because the shapes of the 

 coils lent themselves to the use of the con- 

 venient formula, readily expressible in ellip- 

 tic integrals, for the force, F, between a 

 uniform cylindrical current sheet and a co- 

 axial helix, viz : 



F=rn{M,-M.,), 



where ■/ is the current per unit length of 

 the current sheet, y,^ the current in the 

 helix, and J/j and M^ the coefficients of mu- 

 tual induction of the helix and the circular 

 ends of the current sheet.* 



The value of a particular current of about 

 0.63 ampere having been determined abso- 

 lutely by means of this apparatus, the rate 

 at which it would deposit silver under spec- 

 ified conditions was ascertained indirectly, 

 by observing its silver value on a Kelvin 

 balance which had been kept screwed down 

 in a fixed position for several years past 

 and which had been calibrated many times 



* Proceedings of the Koyal Society, Vol. 63, "On 

 the Calculation of the Coefficient of Mutual Induction 

 of a Circle and a Coaxial Helix, and of the Electro- 

 magnetic Force between a Helical Current and a Uni- 

 form Coaxial Circular Cylindrical Current Sheet , " 

 by Professor J. V. Jones. 



