225 
of Edinhitrgh, Session 1882-83. 
Monday, 2nd July 1883. 
Mr ROBEET gray, Vice-President, in the Chair. 
The following Communications were read : — ■ 
1. Note on the little h group of lines in the Solar Spectrum, 
and the new College Spectroscope. By the Astronomer- 
Royal for Scotland. 
2. On Superposed Magnetisms in Iron and Nickel. By 
Professor C. G. Knott, D.Sc. 
i^Ahstvad.) 
When an iron wire is magnetised longitudinally, it lengthens in 
the direction of magnetisation, according to an old discovery of 
Joule’s. More recently Wiedemann showed that when wire is at 
the same time magnetised circularly it tends to twist. Thus, if an 
iron wire be fixed at one end, and stretched vertically by means of 
a mass attached to the free end, the free end will twist round when 
the wire is both traversed by one current and magnetised by another 
which traverses a helix surrounding it. If the wire is magnetised so 
as to have the north pole down, a down current will make the free 
end twist in the direction of the hands of a watch as looked at from 
above. Reversal of either current reverses the direction of twist ; 
reversal of both produces no alteration. Maxwell and Chrystal 
have pointed out that Wiedemann’s phenomenon can be explained 
by means of Joule’s. 
In the experiences that form the subject of the paper this was 
verified, and other peculiarities in the twisting of iron under the 
infiuence of these magnetisations were discovered which tend more 
securely to establish this relation. Thus, when the linear current 
(the current along the wire) was kept constant, the twist reached a 
maximum for an intermediate value of the helical current, this 
critical value being greater for a greater linear current — a result in 
remarkable harmony with the details of Joule’s discovery. 
Again, for different tensions, the twisting under the influence of 
two given magnetisations varied, being (with one or two exceptions) 
