78 Mr. E. A. Lehfeldt on a 



The shear -j- + -j- is given by 

 |4 : I.4(A + O + ?)-^(80 + 8 A + ™) 



+ ^( 5 oc + ™ B ). 



To see the magnitude of this take # = 0, i/ = 0, r = a, then 



dw r/r _ r 10fi + 6{\ + fi) l 

 dy dx~*V 9X+14/A J 



_a 9\+14^' 



which depends on the value of \//jl, but for all known isotropic 



materials differs little from 2«. 



When \/fji = oo , or the material is incompressible, the 



shear is 



5 

 g «. 



When fji/\ = oo , or the stretch-squeeze ratio vanishes, the 



shear is 



15 

 y*. 



When \ = /4 (Poisson's condition) the shear is 



45 



St. John's Coll., Cambridge, 23 

 Oct. 26, 1891. 



VII. A Galvanometric Method of Measuring H. By R. A. 

 Leheeldt, B.A.,B.Sc, Demonstrator at Firth College, 



Sheffield*. 



IN the ordinary magnetometric, and in Kohlrausclr's method 

 of determining the earth's horizontal magnetic force, 

 the magnetic moment or the strength of current is involved, 

 and two observations are necessary to give two equations 

 between which the unknown may be eliminated. If, how- 

 ever, it is assumed that a current can be measured absolutely, 

 the problem of measuring H is simplified. This may fairly 

 be assumed now, for not only are there very reliable direct- 

 reading ammeters to be had, but the E.M.F of the Clark cell 

 * Communicated by the Author. 



