4:94 Prof. S. "W. Holman on Galvanometer Design. 



the averages 57 and 72 found above. The reason for this 

 state of affairs lies probably in the fact that when salts are 

 dissolved in water there is a shrinkage which probably 

 implies a considerable compression of the water and an altera- 

 tion of its compressibility, so that //^ cannot be taken to have 

 the value found in pure water at low pressures ; from what 

 has gone before it would appear that the alteration of ^ is 

 proportional to (M 2 2 Z 2 )* amongst compounds of the same type. 

 It is evident that the compressibility of solutions has been 

 merely touched on here, and that a complete treatment of the 

 subject would demand a knowledge of the characteristic 

 equation for water. The little that has been written here has 

 been brought forward to indicate that there are simple prin- 

 ciples on which a preliminary exploration can be carried on. 



The chief result of the present paper is to show that the 

 surface-tension of aqueous solutions follows the same law T s as 

 the surface-tension of mixed liquids, except that the ratio 

 iAg/dA^Ag)* does not appear to have the value 1 charac- 

 teristic of most mixtures of two liquids, though it has the 

 same value for all compounds of the same chemical type ; 

 it has also been shown that the values for molecular attraction 

 obtained from the surface-tensions of solutions agree with 

 those obtained according to the Kinetic Theory of Solids. 

 Melbourne, March 1895. 



XLV. Galvanometer Design. Waste Space near the Needle. 

 By Professor S. W. Holman*. 



IN this Magazine for July 1890, Messrs. Ayrton, Mather, 

 and Sumpner gave a demonstration to show that in 

 the coil of a sensitive (Thomson) galvanometer the cur- 

 rent in a portion of the central turns affected the needle 

 in opposition to the remaining major part of the turns ; so 

 that this portion should be left out or connected in reversed 

 direction. The space containing these turns was pointed out 

 as being nearly " an oblate spheroid with a polar axis about 

 0*72 of its equatorial diameter, the latter being of course 

 slightly larger than the length of the needle " (p. 65). 



This point which, if correct, would be of practical signi- 

 ficance, becomes less important from the fact that in the 

 ordinary construction much of this particular volume must 

 be left empty for the insertion and free motion of the needle 

 — one reason perhaps why its partial incorrectness has not 

 hitherto been commented upon. Inasmuch, however, as 

 Gray has introduced it into his most valuable ' Absolute 



* Communicated by the Author. 



