60 PROCEEDINGS OF THE AMERICAN ACADEMY. 



sions of the test pieces were obtained by aid of a set of gauges which 

 had been tested against two comparators, one by Zeiss. The test 

 rods received first a very thin coat of varnish and then the test coil 

 of triply silk-covered copper wire which was in turn varnished and then 

 heated in a stream of hot air until the whole was thoroughly dried. 

 Usually two test coils were wound upon each rod and their indications 

 compared, lest an injury or imperfection in one might escape notice. 



Our first experiments were not so carefully carried out as the later 

 ones, but they pointed to the same general conclusions. I need 

 mention only two of them. In the first, we tested a rather short 

 piece of cold-rolled shafting 1.269 cms. in diameter. This had origi- 

 nally a length of 79 diameters, but was cut shorter by steps to 63, 47, 

 32, 24, and 16 diameters, respectively. In an exciting field of 1280 

 gausses, the fluxes through the central section of the specimen seemed 

 to be as 176, 175, 175, 175, 175, and 175. 



The second experiment was made upon a rod of Norway Iron 1.110 

 cms. in diameter and successively 150, 130, 110, 70, and 30 diameters 

 long. In this case the value obtained for the flux through the central 

 section at the last step differed by less than one eighth of one per cent 

 from the average value for the other steps, and as it happened this 

 small difference was in excess. The truth is that all the values 

 agreed within the small accidental error to be expected in the work. 

 The magnetizing field had an intensity of 2700 gausses. 



We were at first puzzled by a phenomenon which sometimes affected 

 our results by a small fraction of one per cent. After a long day's work 

 when the originally long rod under examination had been cut down, 

 by a succession of steps, to a short one, and the resistance of the 

 circuit had become a little greater than at the outset on account of 

 the heat set free in it, we often found the same flux at very high exci- 

 tations through the central section of our specimen which a slightly 

 greater current had caused in the longer piece in the morning. This 

 was not caused by a rise of temperature in the test piece because a 

 vigorous flow of water of practically constant temperature was sent 

 through the tube of the solenoid all day. It was not due to a change 

 of sensitiveness in the ballistic galvanometer, as frequent calibrations 

 showed. 



It seems to be true that if a rod of soft iron of any length be repeat- 

 edly magnetized in a very strong field, in the one direction and in the 

 other, alternately, a very slightly weaker field will eventually suffice, 

 after it also has been many times reversed, to magnetize the iron as 

 strongly as the original one. 



