Page:Proceedings of the Royal Society of London Vol 60.djvu/109

94 iron was, therefore, under these circumstances, case-hardened, and will he referred to as the hardened iron ring. Having been formed into a transformer in the above-described manner, a long series of observations were taken with this coil to determine its permeability at different temperatures and with different magnetising forces. The results of these observations are shown in the Table V below, and are delineated graphically in the curves in fig. 2, marked Hardened Iron. The results show in a remarkable manner that the iron so treated undergoes a very considerable increase in magnetic permeability when it is cooled to the temperature of liquid air; for certain magnetising forces the permeability at the lowest temperature reached may be increased as much as five times. In this respect, therefore, this iron presents in an exaggerated degree the same qualities found in the unannealed iron.

Table V.—Variation of Magnetic Permeability with Temperature of Hardened Iron. Temperature measured in platinum degrees by standard thermometer P1# Temperature. Perm eability. II = 2-66. H = 4-92. II = 1116. II = 127-7. 0° 56-0 106 -5 447-5 109-0 - 20 5 7 '0 109 5 476-0 108 -5 - 40 58-0 114 0 506 -5 109 0 - GO 59 -0 119 '8 540-0 110-5 - 80 62 5 132 5 575 -0 111-0 - 1 0 0 65-5 150 0 610 -0 112-0 -1 2 0 69-2 169-3 645 0 112-0 -1 4 0 75-3 192 5 680 -0 112-3 - 1 6 0 8 9 - 5 236-0 717-0 114-0 -1 8 0 107 '5 338 -0 762-0 119-5 -2 0 0 132 0 502 -0 823-0 124 0

We have also examined the behaviour of a ring coil made of steel pianoforte wire. We have found in this case the curious result that pianoforte steel behaves in the same manner as the annealed soft iron; its permeability is decreased as the temperature is lowered. The results of the measurements with this steel-core ring are shown in Table VI, and graphically in the curves in fig. 2, marked steel.