170 A. M. Mayer—Effects of Magnetization 
An opaque circular screen is to be placed around the edge of the 
disc so as to cut off all stray light, which I found in my experi- 
ments very annoying. The observer placed behind the ground- 
glass will measure simply with a pair of compasses the length, 
etc. of the streaks, in the manner described by me in another 
place,* and the difficulty of accommodating the eyes for the 
image wiil in great part vanish, from the circumstance that the 
hands will be resting on the ground-glass where the images are 
expected. It is scarcely necessary to add, that to use this or ay 
method successfully, will require some previous training, whicl 
I think could best be obtained by a repetition of the exper 
ments and measurements described by me in a former number 
of this Journal.* 
New York, Dec. 28th, 1872. 
ArT. XX.—On the effects of Magnetization in changing the Di- 
mensions of Iron, Steel and Bismuth bars ; and in increasing the 
Interior Caparity of Hollow Iron Cylinders; by ALFRED M 
Mayer, Ph.D., Professor of Physics in the Stevens Insti- 
tute of Technology. 
Parr I. 
(Read before the National Academy of Sciences, in Cambridge, Nov. 22, 1872.) 
I PURPOSE giving, in a series of papers, the results of a pro- 
longed and careful research on the above subject. 
Introduction.—In 1842 Joule discovered that when a current 
of electricity was passed through a helix which enclosed a bar 
of iron, the latter, on its magnetization, suddenly elongated a 
minute fraction of its length. 
To present clearly Dr. Joule’s experiments, we will give these 
abstracts from the excellent paper which he published in the 
Philosophical Magazine in 1847. Se 
‘‘In order to ascertain how far my opinion as to the invarl- 
ability of the bulk of a bar of iron under magnetic influence was 
well founded, I devised the following apparatus. Ten copper 
wires, each 110 yards long and one twentieth of an inch In 
and 1 inch in diameter. One end of the tube was hermetically 
sealed, and the other end was furnished with a glass stoppe! 
which was itself perforated so as to admit of the insertion of 4 
capillary tube. In making the experiments, a bar of annealed 
iron, one yard long and half an inch square, was placed in the 
* This Journal, II, vol. iv, Oct., 1872. 
