1034 
PROFESSOR J. A. EWING AND MISS H. G. KLAASSEN 
by reference to Foucault currents is not easy to answer. It appears possible that 
another cause may contribute to produce time-lag. In the upsetting of a group of 
molecular magnets time is needed for the disturbance to gradually spread from the 
point, or points, where the mutual controlling forces which produce equilibrium are 
first overcorne. Let a group of pivotted compass needles, for example, be subjected 
to a gradually augmented deflecting magnetic force; the first to yield will in general 
be some outlying member ; its yielding will reduce the stability of its neighbours, and 
the disturbance spreads to them and from them to others. In a bar of iron we may 
conceive the molecules next the surface to be less strongly controlled than those in 
the interior, and to be, therefore, more likely to act as origins of disturbance when 
the equilibrium is upset by application of magnetizing force. From this point of 
view, the greater amount of surface a mass of iron presents the more c^uickly it is 
likely to take uj) the full amount of magnetic induction proper to any applied magnetic 
force, and hence in laminated iron the influence of time-lag should be less conspicuous 
Fig. 34. 
Magnetic curve-tracer curves of steel. 
(a) Cycle performed slowly. 
(b) Period of cycle 3 seconds. 
(c) Period of cycle 0'6 second. 
than in a solid piece. To distinguish an effect of the kind here suggested from the 
effect of Foucault currents is evidently difficult, since both effects depend on the 
metal being tested in relatively large solid masses. A possible exjoeriment might be 
arranged in which a rod built up of a number of concentric cylinders should be 
compared with an equal rod of the same iron built up of a number of sectors parted 
by radial planes. Both modes of lamination would reduce whatever part of the time- 
lag is due to the cause just suggested, but the second mode only would be effective 
in preventing Foucault currents. 
It is clear from the experiments of fig. 33, and even those of fig. 34, that ordinary 
ballistic tests are liable to serious error when the specimen tested is a solid bar of any 
