.370 
MR. C. CHREE OX THE EFFECTS OF PRESSURE 
)nental value. The value of k{ cannot, of course, exceed that of ; so either the 
ordinates of c and d must be rapidly approaching second maxima, or else the ordinates 
( f the curves h and c of fig. 4 must possess minima values in fields lower than were 
experimented on. 
§ 50. In comparing the amounts of the residual and induced magnetisations, the 
employment of the term retentiveyiess in an exact way will be found serviceable. It is 
here used in accordance with the following definition ;— 
The retentiveness is the ratio of the residual to the induced magnetisation, both quan¬ 
tities being measured with the rod exqoosed to one and the same state of mechanical 
stress, and the stress remaining constant during the interred that elapses between the 
measurements. 
In fig. 12 the ordinates give the retentiveness of the rod in the cases illustrated by 
the curves b, c, d. As the residual magnetisation alone was observed in the experi¬ 
ments recorded in Tables VI., VII., and VIII., the necessary values of the induced 
magnetisation were derived by interpolation from Tables IX. and X. In curve b the 
induced magnetisation is that existing prior to the application of pressure, or is the of 
Table IX. In curve c the induced magnetisation is ^2 5 for after six pressure cycles 
the increase in the magnetisation accompanying further pressure cycles is, for our 
present purpose, quite negligible. In the experiments answering to curve cl, the rod> 
after experiencing six pressure cycles, was under pressure wdien the current was 
broken. Thus, for curve cl the induced magnetisation is got by adding to ^2 
algebraic value of the cyclic “ 07i” — “off'' of Table X. 
The curve b sho^vs that, in the absence of pressure, the retentiveness attains a 
maximum in a field of about 15 C.G.S. units, a field somewhat below that at which 
the maximum residual susceptibility occurs in the corresponding curve of fig. 11. A 
great similarity exists between curve b and the curve for zero load in Professor 
Ewing’s fig. 57. The principal difference is that the field at which the maximum 
retentiveness appears is considerably higher in cobalt than in iron. 
The curves c and d of fig. 12 bear a considerable resemblance to the corresponding 
curves of fig. 11 ; but there are no longer distinct maxima or minima. Comparing 
these curves with cmwe b we see that in the weakest experimental fields pressure 
cycles increase the retentiveness in the ratio of four or five to one. The tendency to 
become tangential to the vertical axis shown by the curves c and d —wdiich cannot, 
however, go on indefinitely as the fields are further reduced—is not exhibited by any 
of the curves for the retentiveness of loaded iron-wdre occurring in Professor Ewing’s 
fig. 57. Possibly if he had employed lownr fields he might have found similar 
phenomena, so it -would not be safe to assume that iron and cobalt actually differ in 
this particular. 
The ordinates of curve cl being greater in wnak fields than those of curve c, it 
follows that in fields below 30 C.G.S. units, where the curves cross, the retentiveness 
of the rod is greatest when it is under pressure while the current is broken. In fields 
