Page:Proceedings of the Royal Society of London Vol 60.djvu/105

90 with certain conclusions of Messrs. Laws and Warren (see ‘ Proceedings of the American Academy of Sciences,’ vol. 30, p. 490). These observers made a series of experiments on a material which was practically a very soft steel, and employing a differential watt meter, measured the hysteresis loss in the iron at varying and increasing temperatures up to 600° or 700°. They found that the hysteresis loss in this material did not begin to decrease sensibly until about 150° C .; after that it decreased regularly in accordance with the simple linear function of the temperature. In one experiment which they tried with the same material cooled to —78° C. in solid carbonic acid and ether, they found no difference between the hysteresis loss of this soft steel at that temperature and at the ordinary temperatures. Our results, which have been carried to the much lower temperature of liquid air, indicate that in the case of soft annealed Swedish iron the hysteresis loss is not changed by cooling from ordinary temperatures to the temperature of liquid air. As we kuow that the hysteresis loss of soft iron decreases when the temperature is increased, from the ordinary experience with transformers in commercial use, the matter that requires further investigation is to discover the temperature at which the hysteresis loss sensibly changes and begins to diminish.

We have also carried out a series of experiments of the same character with unannealed iron and steel. A ring coil was constructed of sheet iron of the same quality as that forming the core of the soft iron transformer above described, but no special pains were taken to anneal the iron, and as it was “ hardened ” in a magnetic sense by being bent into shape, this difference in quality showed itself in the magnetic observations. A ring coil was constructed of the following dimensions:—The thickness of the strip was 0 031 cm., width of the strip P24 cm., the ring was formed by 23f layers of this sheet iron wound up closely into the form of a ring. The outside diameter of this ring was 4 cm., the' inside diameter 2T3 cm., the cross-section of the iron in the ring was therefore 0'9032 sq. cm., and the mean perimeter of the ring 9-62 cm. This iron ring was not annealed in any way, but it was simply wound over with silk ribbon, and then had placed upon it two coils of wire. The primary coil consisted of 150 turns of No. 26 wire, having a resistance of 0 383 ohm, and the secondary coil consisted of 240 turns of No. 36 wire having a resistance of 8‘092 ohms. As the diameter of crosssection of the ring was not very small compared with the mean diameter of the ring, it was necessary to calculate by a proper integration the mean value of the mean magnetising force in terms