Permanent Magnetic Field, 313 



point, and found it very small, but as the tests were made 

 with hastily-adjusted appliances, I have thought it better to 

 reserve the matter for further investigation. The temperature 

 range involved in the Table is 10° C, but the figures cannot 

 be used for deducing the temperature-coefficient because of 

 the other sources of error. I am inclined to think that the 

 temperature variation of the condenser (which has not been 

 applied to the calculations) practically neutralizes that of the 

 inductor. 



In addition to the evidence of constancy afforded by the 

 Table, there are in my note-books several facts which point 

 the same way. Of these I shall mention only one. 



It is well known that magnetic decay is most pronounced 

 just after magnetization, especially if the magnet is subjected 

 to vibration. In several early cases I tested the effect of 

 vigorous blows during and immediately after magnetization, 

 but the evidence of loss was generally very feeble, and in 

 some cases not measurable. In this connexion it may be 

 worth noting that the brass ring which carries the coil is fairly 

 heavy, and that when it falls it produces an appreciable blow. 

 Each of the three instruments has been subject to this shock 

 hundreds of times, but has shown no sign of decay under it. 



Perhaps I may be permitted to say that I do not advance 

 the principle of the magnetic circuit as at all novel. It has 

 already been embodied in several well-known applications. 

 But I believe the idea is here made subservient to new 

 purposes. 



As a material embodiment of a standard of magnetism 

 (magnetic lines, gausses, or whatever else may be the right 

 name) I find it helpful to many students. For this reason I 

 propose to adjust future instruments of this size to a round 

 number of lines, say 20,000 or 25,000, which will facilitate 

 calculations arising out of their use. 



The instrument is a most convenient standard for measuring 

 magnetic quantities, whether it be the lines in any other 

 magnet or the vertical and horizontal components of the 

 earth's field. I am trying to use it for developing a new 

 method of determining these quantities. 



It enables me to simplify the magnetometer method for 

 determining magnetic permeability. Over the ordinary mag- 

 netizing coil I wind a sufficient number of turns of a secondary. 

 The coil with its core is fixed in any position that is convenient, 

 and the relative number of lines determined by the mag- 

 netometer deflexion in the usual way. At the end of the 

 magnetization the total absolute number of lines (corresponding 

 to the maximum magnetometer deflexion) is found by taking 



