402 Proceedings of Eoyal Society of Edinburgh. [sess. 
over a scale at a distance of 1*7 metres from tlie magnetometer 
mirror. Before the wire was inserted, the electromagnetic action of 
the coil on the magnetometer was corrected by a small adjustable 
coil in circuit with the solenoid and set close to the magnetometer. 
The wire was then inserted and subjected to cyclic variations of 
magnetic force. The magnetising current was changed continuously 
from a given positive value to an equal negative value, and back 
again to the original positive value. The variation was effected in 
a gradual manner by means of a liquid rheostat, consisting of a 
column of dilute sulphate of zinc with zinc electrodes, through 
which a steady current from a battery of Daniell cells was con- 
stantly flowing. By means of a sliding zinc electrode, the necessary 
current was shunted through the solenoid circuit. The tangent 
galvanometer included in this circuit was, of course, far removed 
from the solenoid, and was so placed that the operator in adjusting 
the rheostat could easily read the galvanometer deflection. 
In the earlier experiments careful attention was paid to the first 
effects, as well as to the permanent cyclic condition, which soon 
becomes established after a few cyclic changes of the magnetising 
current have been gone through. At suitable stages in the variation 
of the current, the current was kept steady until the corresponding 
deflection of the magnetometer needle was observed and noted. 
These remarks will suffice at present to indicate the general method 
adopted in studying the cyclic changes which have been so fully 
investigated by Warburg and Ewing. 
One of my objects was to study the modification produced on 
this cycle, and on whatever else may be associated with it, when a 
current is passed along the magnetised wire. This linear current 
(as we shall call it) was derived from one or more Bunsen cells. It 
entered the iron wire at the end furthest removed from the magneto- 
meter, and returned along two copper wires stretched parallel to the 
iron wire, and very close to it. 
A complete set of experiments consisted in — 
(1) Taking a permanent magnetic cycle when no linear current 
was flowing along the wire ; 
(2) Observing the initial effect when the linear current was 
made to flow in one direction along the wire, and taking the 
permanent magnetic cycle, with this current kept steady ; 
