ON THE MAGNETISATION OP COBALT. 
335 
be brouglit into the critical state by much weaker currents than the portions under 
the ends of the spiral. 
In his § 240, Sir W. Thomson states that a Villaph critical point was found in the 
central portions of a nickel bar, but from a note to § 239, it would appear that with 
the magnetometer opposite an end of the bar, no trace of a neutral field was obtained. 
With a magnetometer as usual opposite one end, he found in nickel and cobalt 
a cyclic effect the reverse of that occurring in iron in fields below the Villaui point, 
i.e., the mao;netisation was least when tension was “on.” 
§ 10. Mr. Shida obtained Villari critical fields of 15 and 10 C.G.S. units respec¬ 
tively for two different specimens of soft iron wire, which had been permanently 
stretched, and on which the tension cycles were performed with weights not much less 
than those causing the permanent extension. With a pianoforte steel wire, he found 
that in the cyclic state tension “ on ” co-existed with a minimum of magnetisation in 
all his fields. 
§ 11. The first thing in Professor Ewing’s researches that concerns us is his method 
of demagnetising wires. This consists in subjecting them to the action of a series of 
rapidly reversed currents diminishing in intensity. In § 19 of the first of his papers 
referred to above, he states that his wires v/ere thereby reduced to a standard 
condition—different possibly, he admits, from their condition previous to their first 
maguetisation. 
In his § 107 Professor Ewing defines the Villari critical point as “ that value of 
the magnetisation 3 at which reversal occurs in the sign of the magnetic difference 
produced by two (assigned) states of longitudinal stress.” He uses the term as 
applicable (1) to that value of 3 at which no magnetic change accompanies the 
repeated alternation from one to the other of two assigned states of stress, (2) to the 
value of 3 answering to the intersection of the two curves giving the relations of 3 to 
—the field,—when is gradually I’aised from zero, and separate experiments are 
completed with the wire in each of the assigned states of stress. He also proposes 
that one of the assigned states of stress should be that answering to no load. It will 
be noticed that Professor Ewing’s first usage of the “ Villari critical field” is that 
which accords with Sir W. Thomson’s. 
In originally soft annealed iron wires stretched beyond the limit of perfect 
elasticity. Professor Ewing found in every case distinct Villari points of the first 
kind. These existed in lower fields the greater the weight producing the second 
assigned state of stress. If in a certain field the cyclic state has been reached for the 
loads 0 and Q, for which the magnetisation has the critical value, and the load be 
gradually raised from 0, then there ensues a rise in the magnetisation until the load 
reaches some value P less than Q, and then an equal fall as the load is increased to Q. 
Even in the weakest fields Professor Ewing seldom found the magnetisation increase 
continually when the load was raised nearly to the limit of perfect elasticity. 
From the figures on Professor Ewing’s Plate 63, it will be seen that the first 
