Chemistry and Physics. 559 



According to Ampere an electric current is associated with a 

 magnetic field equivalent to that of a magnetic shell whose mag- 

 netic moment in is equal to the product of the current i and the 

 area of the enclosed surface F. In the case of a charged particle 

 moving around the circumference of a circle the current is equal 

 to the product of the number n of revolutions per second by the 

 charge e of the particle, that is, m = iF = e n F. Considered 

 vectorially, the magnetic moment is perpendicular to the plane of 

 the orbit and is related to the direction of circulation of the ele- 

 mentary charge according to the right (positive charge) or left 

 (negative charge) hand screw law. Again, as may be readily 

 shown, the angular momentum M of the charged particle of 

 Newtonian mass /x is given by the equation M = 2 fj. n F., con- 



'lix, 

 sequently M = — m. This relation may be extended to all the 



electrons and molecules in a body of ordinary size, and it then 

 assumes the form M = — 1*13 X 10 -7 J, where J denotes the vol- 

 ume integral /m dr. Finally, we obtain D = 1*13 X 10~ 7 v X J 

 in which D and v symbolize respectively the mechanical moment 

 and the angular velocity of the extended magnetic body. This 

 moment is analogous to that which governs the motion of preces- 

 sion in the theory of gyroscopes. The rest of the analysis con- 

 sists in deriving equations for the motion of the fundamental 

 parts of the apparatus employed. 



The experimental investigation was divided as usual into two 

 parts, (a) the qualitative verification of the phenomena predicted 

 on the foregoing theory, and (b) a more exact quantitative test of 

 the theoretical deductions. Although the first part of the work 

 is very interesting and leaves no doubt concerning the validity of 

 the theory, we shall turn our attention at once to the final experi- 

 mental procedure. 



The perfected appai'atus consisted of two fundamental parts, 

 namely, a hollow coil of insulated wire with its axis of figure 

 vertical, and a slender rod of iron suspended coaxially in the coil 

 by means of a fine glass fiber. In order to completely reverse the 

 magnetization of the iron rod (as the theory presupposes) the 

 dimensions of the apparatus had the following values. The coil 

 was 62 cms long and had about 100 turns per cm. A current of 

 T45 amperes produced fields of 260 and 130 gausses at the middle 

 and ends of the coil, respectively. To minimize the demagnetiz- 

 ing effect of the poles the iron rod was made 16 cms long and 0'l7 cm 

 in diameter. A very thin glass tube, which was cemented to the 

 lower end of the iron rod, projected below the coil and carried a 

 tiny mirror which reflected the "light lever" and thus enabled 

 the experimenter to observe the amplitudes of the rotations of the 

 rod around its vertical axis. By varying the frequency of the 

 alternating current in the coil and noting the corresponding 

 angular elongations the data for a resonance curve were obtained. 

 This curve has the general appearance of a slightly asymmetric 

 " error curve " with such steep sides that the maximum can be 



