in Magnetic Moment produced hy Small Forces. 449 



Weber's method becomes more convenient and more exact 

 if, instead of reversing the magnet and spiral with reference 

 to the earth's magnetism, we reverse a magnetic force with 

 reference to the spiral and the magnet in the spiral. For this 

 purpose the induction-spiral was further wound with 280 turns 

 of wire, forming a coil of 5 centim. diameter and a total 

 length of 89 centim., through which a current was passed, 

 producing within the spiral a magnetic field of about 0*2 C.G.S. 

 unit, and thus of abo'jt the same strength as the earth's hori- 

 zontal magnetism. This current was made or interrupted so 

 as to increase or to diminish the magnetism of the bar. Mo- 

 mentary currents were thus induced in the inner spiral, which 

 were measured by the method of multiplication. Moreover, 

 the part of the induced current resulting from the magnetizing 

 current itself could be compensated by means of a second 

 duplicate spiral, which is an advantage of this electromagnetic 

 method. A similar arrangement has already been described 

 by Topler and Von Ettingshausen*. Local disturbances are 

 eliminated by reversal of the current and magnet. 



In order to infer the induced magnetism from the deflec- 

 tions of the galvanometer, a small magnet of known moment 

 was employed , by the use of which experiments according to 

 the method of multiplication could be made by introducing 

 the magnet into the spiral and withdrawing it. 



The steel bars were examined in various conditions of hard- 

 ness and intensity of magnetization, as detailed in the first 

 column of the following Table. The mass m of the bars is 

 given in grammes, the magnetic moment in centimetre-gramme 



M 

 units, and under s the specific magnetism — or the magnetic 



moment per gramme of steel. m 



Lastly, under + As and — As are given the specific induc- 

 tion constants ; so that the numbers denote the change in 

 specific magnetism ; in other words, the increase or decrease 

 in the magnetic moment of the unit of mass (1 gr.) which 

 is produced by the unit of magnetizing or demagnetizing 

 force (gr.* cm. - * sec. -1 ). 



By way of explanation, it may be observed that the number 

 As represents nearly the change in specific magnetism pro- 

 duced in our latitudes by reversing the bar in the direction of 

 the dip-needle. About the fifth part is the u magnetism of 

 position " produced in 1 gr. in our latitude by the earth's 

 horizontal magnetism f • 



* Pogg. Ann. clx. p. 1 (1877). 



t Multiplied by 7*() (the density of steel) the numbers s or As give the 

 specific magnetism or induction-constant (as the case may be) referred to 

 unit volumes, which latter, as Herwig has remarked, represent absolute 

 numbers, and are thus independent of the units employed. 



