Certain Seasoned Hard Steel Magnets. 247 



of non-magnetic steam and cold water pipes, furnished with 

 suitable valves and connections, the tank, which had a good 

 stirring apparatus, could be filled with about 15 liters of water 

 at any required temperature from about 15° C. to 100° C. 



On the opposite side of the magnetometer was a seasoned 

 compensating magnet in Gauss's A position, held, and pro- 

 tected completely from rapid temperature changes, within a 

 wooden carriage sliding on a horizontal scale. This compen- 

 sating magnet having been placed with its center at a con- 

 venient distance (d ) from the center of the magnetometer 

 needle, the tank, filled with water at nearly the temperature of 

 the room, was fixed at such a distance from the magnetometer 

 as to bring the needle back into the meridian. If M is the 

 magnetic moment of the compensating magnet and %l its mag- 

 netic length, and if M x is the moment of the magnet to be 

 tested and M/ its moment when, having been heated to a 

 higher temperature, the needle is deflected so as to cause a 

 scale reading n with a scale distance a, 



M-M 1 = }In K 2 -C) 2 

 M x M ' 4ad 



If a a is the angle through which M would deflect the needle 

 if M x were absent, 



M 1 — M/ _ n 

 M x — 2atana 



The method of procedure was then as follows: The com- 

 pensating magnet was placed at a given fixed distance from 

 the magnetometer, the magnet to be tested clamped in the 

 non-magnetic holder in the tank, and cold water turned in. 

 The position of the wooden tank was then adjusted until the 

 needle showed its original zero reading. Steam was then 

 passed in at intervals and deflections ^taken at various desired 

 temperatures up to 100°, the positions of both magnets remain- 

 ing unchanged throughout the process. The dimensions and 

 the magnetic moment of the compensating magnet being 

 known, the fractional loss of moment of the magnet to be 

 tested could be easily calculated from the formula given above. 

 With this apparatus I tested about 25 magnets; additional mag- 

 nets were tested for me in a similar manner by other observers. 

 The total number tested was sufficient to determine with some 

 degree of certainty how much the curve of variation of mag- 

 netism differed from a straight line. 



It appeared in the first place that among the magnets which 

 had not been heated above room temperature for a long time, 

 some did not regain their original strengths immediately after 

 being heated to 100° C. and then cooled to room temperature 

 again. There was in several instances a temporary loss of moment 



