bi StON] 
XLVIIIL—NOTE ON THE CONDUCTIVITY OF TOURMA- 
LINE CRYSTALS, sy GEORGE FRANCIS FITZGERALD, 
M.A., F.T.C.D. 
{Read January 19th, 1880. ] 
In the Philosophical Magazine for July, 1879, Professor Sylvanus 
Thompson and Dr. Oliver Lodge give the results of some very 
interesting experiments upon the unilateral conductivity of tour- 
maline erystals for heat and electricity. Dr. Lodge had shown 
that an explanation of pyro-electricity might be given if such 
crystals possessed a unilateral conductivity for electricity. A 
body is said to possess unilateral conductivity for anything if it 
conducts better in one direction than in the opposite one ; as, for 
example, a tube with a series of funnels in it all turned the same 
way for fluids, and apparently in the case of Geissler’s tubes for 
electricity also. The result of their experiments was that tour- 
maline crystals do possess a unilateral conductivity for heat as 
long as their temperature is variable and similarly for electricity 
as long as the temperature varies. The first of these facts is an 
important and valuable increase of our knowledge, but the latter 
as they point out is of course only due to the already known 
electromotive force which constitutes their pyro-electric properties. 
They seem to have been dissatisfied with these results, for they 
had hoped to discover unilateral electric conductivity inde- 
pendently of changes of temperature. They do not seem to have 
noticed that what analogy should have led them to look for was 
unilateral conductivity during changes of intensity of the 
current. It is to be hoped that, as they possess a very fine 
specimen of tourmaline, they will continue their investigations 
into this point. In the meanwhile it may be worth noticing a 
mechanical illustration of how this might be connected with 
pyro-electricity. Suppose a wire carrying a current, surrounded 
by a number of magnets, and that a majority of them pointed 
in one direction round the wire, and that each was kept in its 
_place by a spring. On passing a current through the wire, 
all the magnets that did not point round it in a particular 
direction would tend to set themselves in this direction, and 
