88 Prof. E. Bouty's Studies on Magnetism. 



N, for a great number of values of N. For the intermediate 

 values proportional parts are taken, 



The masses of iron distributed, in fixed positions, in the labo- 

 ratory where the operations are carried on, have no influence on 

 the measurements, provided that they are not too near the appa- 

 ratus ; they merely modify the azimuth of equilibrium, which 

 we take for our magnetic meridian. 



In short, a space of ten minutes is sufficient for accomplishing 

 a good experiment ; and the result of several successive measure- 

 - ments of the same needle furnishes, in general, numbers equal 

 within 200 °^ tne ^ r value. Moreover the operation becomes so 

 simple and convenient with practice, that we prefer to employ 

 this method even in those cases in which the oscillation method 

 would give good results. 



Comparison of several Apparatus, and Absolute Measures. — One 

 has often to compare several apparatus. This is accomplished 

 by effecting the measurement of the moment of one and the 

 same needle by means of two apparatus which we wish to com- 

 pare. Another needle, suitably selected (larger, for instance), 

 will permit the comparison of the second apparatus with a third, 

 and so on. 



Further, all the relative measures can be converted into abso- 

 lute measures : for this it is sufficient to determine, by the oscil- 

 lation method, the magnetic moment of a needle, and afterwards 

 measure it by means of the apparatus which have been compared. 



II. On Magnetization by Currents. 

 1. On Extra Currents. 



A coil of conducting wires possesses magnetic properties as 

 long as it is traversed by a current. During the time occupied 

 in establishing the current the production of this magnet absorbs 

 a certain quantity of work in addition to that which would be 

 necessary to establish the current in an equal linear resistance. 

 This absorption of work is manifested outside the coil by the in- 

 verse extra current. 



When the current is interrupted, the destruction of the mag- 

 netic property of the coil restores the work absorbed in its pro- 

 duction ; hence the direct extra current, equal in quantity to the 

 inverse. As experiment shows that the first is shorter in dura- 

 tion than the second, we can affirm that the magnetic property 

 of the coil is lost more quickly than it is produced. 



"When a coil of conducting wires is furnished with a core of 

 soft iron, the extra currents retain their character, but are much 

 augmented in intensity. This augmentation measures the work 

 absorbed by the magnetization, or restored by the demagnetiza- 



