108 Propagation of Magnetisation of Iron. [May 31, 



V. " Propagation of Magnetisation of Iron as affected by the 

 Electric Currents in the Iron." By J. HOPKINSON, F.R.S., 

 and E. WILSON. Received May 17, 1894. 



(Abstract.) 



Consider a solid, cylindrical electromagnet, it is well known that, 

 in reversing the magnetising current, the induction does not instantly 

 reverse, but a certain time elapses before it again attains its full 

 value, that it reverses at a later time at the centre of the core than 

 near its surface, and that the delay in reversal near the centre is due 

 to the electric currents induced in the iron. The object of the pre- 

 sent paper is to investigate these effects. 



The magnet experimented upon had a diameter of 4 inches, and 

 formed a closed magnetic circuit. Through a part of its length the 

 cylinder of 4 inches diameter was formed of an iron core surrounded 

 by two concentric, closely fitting tubes. Exploring coils of fine 

 copper wire were bedded in the iron between the surfaces of the tubes. 

 The currents induced in these exploring coils were observed when the 

 current in the main coil of the magnet was reversed. These currents 

 in some cases last for over half a minute. 



Inferences can be drawn from these results as to the behaviour of 

 other diameters than 4 inches. Comparing two cylinders of different 

 diameters, similar events occur, but at times proportional to the 

 squares of the diameters of the cylinders. From this consideration 

 and the experiments, a judgment is formed as to the effects of local 

 currents in the cores of transformers and of the armatures of dynamo 

 machines. 



VI. " On Rapid Variations of Atmospheric Temperature, espe- 

 cially during Fohn, and the Methods of observing them.'' 

 By J. Y. BUCHANAN, F.R.S. Received May 29, 1894. 



The variation of the temperature of the air in the course of a day 

 is a matter of familiar observation. It depends in the first instance 

 on the relative positions of the locality and the sun. The temperature 

 is generally highest a short time after the sun has attained its greatest 

 altidude above the horizon, and it is lowest some time after it has 

 attained its greatest depression below the horizon. Observations 

 made at regular intervals over the twenty-four hours show a more 

 or less regular rise of temperature during the early part of the day 

 and a similar fall of temperature during the latter part of the day 

 and the evening. When the interval between the observations is 



