1886.] Magnetisation and the Length of Iron Wires. 257 



The svstem of the sensory supply is examined in detail. It is shown 

 to follow a law composed of two rules : — 

 III. A. Of two spots on the shin, that which is nearer the preaxial 

 border tends to be supplied by the higher nerve. 

 B. Of two spots in the preaxial area, the lower tends to be supplied 

 by the lower nerve, and of two spots in the postaxial area, 

 the lower tends to be supplied by the higher nerve. 



It is shown that this is the case with all membranes stretched into 

 a sheath by something pushing out into them, and the epiblastic layer 

 of the epidermis is compared to such a membrane, pushed into a tubal 

 sheath by the developing mesoblast. 



A note is added showing that other observers have reached similar 

 results by other methods, and notably that Forgue has formulated 

 laws for the motor nerves of the monkey, identical with those laid 

 down in the present paper. 



II. " On the Changes produced by Magnetisation in the 

 Length of Iron Wires under Tension." .By Shelford 

 Bid well, M.A., LL.B. Communicated by Professor F. 

 Guthrie, F.R.S. Received March 10, 1886. 



In a paper communicated to the Royal Society about a year ago,* 

 I discussed the results of certain experiments made by Joule in rela- 

 tion to " the Effects of Magnetism upon the dimensions of Iron and 

 Steel Bars."t 



It is well known that the length of an iron rod is in general slightly 

 increased by magnetisation. Joule enunciated the law that the elonga- 

 tion is proportional in a given bar to the square of the magnetic 

 intensity, and that it ceases to increase after the iron is fully saturated. J 

 My own experiments, made with a greater range of magnetising forces 

 and with thinner rods than those used by Joule, show that if the 

 magnetising current is gradually increased after the so-called satura- 

 tion point of the iron has been reached, the elongation, instead of 

 remaining at a maximum, is diminished, until when the current has 

 attained a certain strength, the original length of the rod is unaltered, 

 and if this strength be exceeded, actual retraction is produced. 



Joule also found that when the experiment was performed upon an 

 iron wire stretched by a weight, the magnetic extension was in all 



* " On the Changes produced by Magnetisation in the Length of Rods cf Iron. 

 Steel, and Nickel." " Proc. Roy. Soc," vol. 40, p. 109. 



t " Phil. Mag." [3], vol. xxx, pp. 76, 225, and the Phya. Soc.'s Reprint of 

 Joule's Scientific Papers, p. 233. 



X Reprint, pp. 2 i5, 255. 



