18 Prof. Thompson and Dr. Lodge on Unilateral 



by 5. My values lie between the limits 



-0-0186 and +0-0281*; 

 Tomlinson's values between 



-0-06 +0-04 (?). 



Subjoining the limiting value from the experiments of Beetz, 



+ 0*0006, 



with which Chwolson's exactly, and Thomson's in some mea- 

 sure, agree, we see that my numbers hold the middle place. 



Tomlinson moreover appears to have carried out the mag- 

 netizing^ while the principal current was closed. I have men- 

 tioned that in such cases an extra current arises, corresponding 

 to a diminution of the resistance. Now, since these extra 

 currents in hard steel often pass very slowly, perhaps a part 

 of the 6 per cent, decrease might result from this, and of the 

 1-4 per cent, increase in soft wires we might deduct a part 

 for the heating acknowledged by Tomlinson himself. 

 [To "be continued.] 



II. On Unilateral Conductivity in Tourmaline Crystals. 

 By Professor S. P. Thompson and Dr. O. J. Lodge! . 



XN thinking of the possible physical conditions of structure 

 which might permit an explanation of the phenomena of 

 pyroelectricity of the tourmaline and other crystals, an hypo- 

 thesis suggested itself independently to each of the present 

 writers. At the Glasgow Meeting of the Association in 1876, 

 a paper was read by the second-named of them, upon a Me- 

 chanical Model illustrating the phenomena of electric currents^. 

 The physical illustrations there given of the relation of elec- 

 tromotive force and resistance to the particles of matter in a 

 conducting circuit led the way to the suggestion that the in- 

 ternal polarization of each particle of the crystal, which had 

 been assumed by Sir William Thomson as a sufficient cause of 

 the phenomena of pyroelectricity, might become explicable if 

 it could be shown that such bodies as possessed pyroelectric 

 properties possessed also a unilateral conductivity for elec- 

 tricity. 



The term unilateral conductivity is defined as follows: — Let 

 a certain direction from a point A to a point B in a homoge- 

 neous substance be considered. Then if it is found that the 



* The value —0-03 has been given by only one experiment with a steel 

 wire, Mn : H being very small and the absolute value of H very great. 



t Read before Section A of the British Association at Dublin, Septem- 

 ber 1878. Communicated by the Authors. 



J See Phil. Mag. Nov. and Dec. Suppl. 1876. 



