472 Prof. W. F. Barrett on certain remarkable Molecular 



It is an evident conclusion that differential galvanometers in- 

 tended for measuring resistances comprised within wide limits, 

 both high and low, should have coils of long fine wire, having 

 necessarily a high resistance ; for a galvanometer with coils of 

 short and thick wire is only suitable for measuring small resist- 

 ances ; whereas if it have coils of fine wire it is suitable for both 

 high and low resistances — for the latter by shunting. 



LXII. On certain remarkable Molecular Changes occurring in 

 Iron Wire at a low red Heat. By W. F. Barrett, F.C.S., 

 Professor of Experimental Physics in the Royal College of 

 Science, Dublin*. 



IN the ' Proceedings of the Royal Society ' for January 28, 

 1869, Mr. Gore published the important fact, that when 

 an iron wire is heated to bright incandescence and then allowed 

 to cool, a momentary elongation, or, as Mr. Gore believed, di- 

 minution of cohesion, of the wire occurs just after it has begun 

 to contract by cooling. The main points in Mr. Gore's paper 

 are as follows : — A thin iron wire fixed at one end to a binding- 

 screw is attached at the other to an index which multiplies any 

 motion of the wire ; the wire is strained horizontally by a feeble 

 spring; and matters are so arranged that the wire can be heated 

 by an electric current or by a row of gas-jets. When heated,, 

 the wire expands and the spring pulls the index over. A sketch 

 of the instrument is given in the Philosophical Magazine for July 

 1869. Mr. Gore states that no anomalous action is observed on 

 heating the wire to bright incandescence ; but when the heating is 

 discontinued and cooling begins, the index moves back until a 

 moderate red heat is attained, when suddenly the pointer gives 

 a jerk or kick, indicating a momentary elongation of the wire 

 during the progress of its contraction. This effect is perfectly 

 certain, and always occurs at this particular temperature. Mr. 

 Gore states that iron wire of a certain thinness and a certain 

 tension of the spring is necessary, and that the phenomenon is 

 apparently confined to cooling iron, no such change being evi- 

 dent during the heating or cooling of wires drawn from the 

 wide range of other metals he has examined. Further, Mr. 

 Gore has investigated the production of induced currents during 

 the cooling of magnetized. iron bars, one portion of which had 

 been heated to redness; and the result showed that the iron bar 

 " suddenly increased in magnetic capacity during cooling at a 

 particular temperature of moderate red heat." 



Having occasion to show Mr. Gore's discovery in the course 

 of a lecture delivered some eighteen months ago to the Dublin 

 * Communicated bv the Author. 



