73 



I. On the Persistency of Electric Currents 



IN Masses of Iron. 



II. The Cause of the Luminosity of Flame. 



By Prof. H. Lamb, M.A. 



The following abstracts form the substance of some remarhs, 

 accompanied by experiments, delivered by Prof. Lamb before the 

 Society on the evening of March 3, 1885, imth supplementary 

 remarks by Prof Mennie : — 



0:s THE Peesistenct or Electric Cueeents iis- 

 Masses of Ieo:n'. 

 Professor Lamb showed some experiments illustrating the 

 gradual decay of electric currents circulating in a cylindrical 

 bar of iron, and subject only to their mutual induction. A 

 bar of iron two inches in diameter was surrounded by two 

 <3oils of copper wire, which may conveniently be called the 

 primary and the secondary coil. A steady current from a 

 battery is made to circulate round the primary coil, and so 

 magnetise the iron bar. This circuit being suddenly broken, 

 currents are induced in the bar, which tend to maintain its 

 magnetism. The decay of these currents and of the accom- 

 panying magnetism produces a gradually diminishing electro- 

 motive force of induction in the secondary coil, which is in 

 -circuit with a sensitive mirror galvanometer. The intensity of 

 this electro-motive force at any instant after the break of the 

 primary may be estimated by closing the secondary circ uit, 

 which is previously open, and observing the first swing of the 

 galvanometer needle. The breaks and makes were made by 

 means of an Attwood's machine, and the interval betwee n the 

 *' break" of the primary and the "make" of the secondary could 

 be varied. In the actual experiments this interval was increased 

 by successive steps of '15 of a second. In this way the gradual 

 decay of the currents in the iron can be traced without diffi- 

 culty for over 1| seconds from the time of the original break. 

 According to theory the persistency of the currents is propor- 

 tional to the square of the diameter of the bar, so that for an 

 iron cylinder a foot in diameter the corresponding interval of 

 time would be about a minute. Some account of the above 

 ■experiments was given to the Society nearly two years ago, but 

 the experiments have recently been repeated with various 

 improvements in the method of observation, and the author 



