102 Barrett, Brown & Haprrenp—On the Electrical Oonductivity and 
resistances R by means of the terminal L. The resistance coils, marked 1 to 6 
in the above fig. 7, had values of 41, 10, 5, 3:8, 0:94, and 0°54 ohms, respectively, 
and were connected with a series of mercury cups. When L was in the position 
shown in the diagram, the whole of the resistances were in circuit; then, by 
inserting the second terminal L’ into the next mercury cup and removing L, the 
magnetising current was suddenly increased without any interruption of the 
circuit. Again, by putting L into the third mercury cup, and removing LU, the 
next step up in the magnetising current was made, and so on, till all the coils 
were cut out and the maximum magnetising current reached; the process was 
then repeated backwards, stepping down till all the resistances were again in, 
when the circuit was broken by removing L. The reversing key R K was then 
thrown over, and a second series of steps up and of steps down were made; the 
current was then once more reversed, and a third series of like steps made; this 
completed the cycle for that one rod. Between each of the steps one observer 
read the deflections on the magnetometer M, and another read the strength of the 
magnetising current as measured by a pair of direct reading Weston ammeters, 
and a Kelvin magneto-static ampétremeter, one of which is shown diagrammatically 
at G. One of the Weston instruments was graduated to read 0:001 ampére per 
division, and the other to read 0:05 ampére per division, while the magneto-static 
read 0:2 ampére per division. ‘These three instruments thus gave us a continuous 
scale measuring from 0:001 ampére up to 18 ampéres, the Weston ammeters 
being switched out whenever the current became too large for their respective 
ranges. Before use, these instruments were carefully tested by means of a Kelvin 
standard ampére balance. 
In order to reduce the rod under test to a non-magnetic state before putting it 
through the cycle, the reversing key was rapidly oscillated by one observer, 
whilst another (by shifting the terminals LL’ up and down the mercury cups) 
threw a gradually increasing or decreasing magnetising current round the 
solenoid; by this means each test-rod could, in a very short time, be reduced to a 
neutral state, as shown by the magnetometer needle returning exactly to its zero 
position. Each rod that was tested was taken through a complete magnetic 
cycle, with a maximum magnetising force of 45 C.G.S. units. This involved 
a total of 36 steps in each experiment. 
The effect of the various series of steps will be better seen by referring to 
fig. 8, where the axes OH and OB represent, respectively, the magnetising force 
and the induction produced. ‘The first series of steps up gives the initial curve 1, 
starting from the origin O; the next series down gives the part 2 from the point 
of greatest induction to where the curve meets the axis OB; the circuit is now 
broken and the current reversed; the next series of steps up gives that part of 
the curve marked 38, 3, cutting the axis O H, and extending down to the other 
