546 



after birth. He has dissected and figured eight or nine of the more 

 important stages, and shown the relative alteration of each part con- 

 secutively, commencing with the Zcea taken from the egg, and pur- 

 sued the observations through the older forms to that of the adult 

 Carcinus. 



The paper is carefully illustrated by drawings made by the author. 



XIX. " On the Electro-dynamic Qualities of Metals : Effects 

 of Magnetization on the Electric Conductivity of Nickel 

 and of Iron." By Professor W. THOMSON, F.R.S. Received 

 June 18, 1857. 



I have already communicated to the Royal Society a description 

 of experiments by which I found that iron, when subjected to mag- 

 netic force, acquires an increase of resistance to the conduction of 

 electricity along, and a diminution of resistance to the conduction of 

 electricity across, the lines of magnetization*. By experiments more 

 recently made, 1 have ascertained that the electric conductivity of 

 nickel is similarly influenced by magnetism, but to a greater degree, 

 and with a curious difference from iron in the relative magnitudes of 

 the transverse and longitudinal effects. 



In these experiments the effect of transverse magnetization was 

 first tested on a little rectangular piece of nickel 1-2 inch long, -52 

 of an inch broad, and '12 thick, being the "keeper" of the nickel 

 horse-shoe ( 143) belonging to the Industrial Museum of Edin- 

 burgh, and put at my disposal for experimental purposes through 

 the kindness of Dr. George Wilson. Exactly the method described 

 in 1 75 of my previous communication referred to above, was fol- 

 lowed, and the result, readily found on the first trial, was as stated. 



The effect of longitudinal magnetization on nickel was first found 

 with some difficulty, by an arrangement with the horse-shoe itself, 

 and magnetizing helix ( 143), the former furnished with suitable 

 electrodes for a powerful current through itself, and the system 

 treated in all respects (including cooling by streams of cold water) 

 as described in 156, for a correspondiug experiment on iron. The 



* See PhiL Trans. Bakerian Lecture, " On the Electro-dynamic Qualities of 

 Metals," Feb. 27, 1856, 146 of Part 4 and Part 5. In the present communica- 

 tion that paper will be referred to simply by the sectional () numbers. 



