May 30, 1902.] 



SCIENCE. 



853 



students, in order to qualify for the M.D. 

 degree, in 26 out of the 156 institutions 

 in the United States was 'over 4,000 

 hours.'* Inasmuch as the minimum re- 

 quirement established by the Association of 

 American Medical Colleges in June, 1899, 

 was 'at least 3,300 hours,' it can be as- 

 sumed that the 26 colleges mentioned above 

 include the institutions of highest standing. 



A laboratory course of physics of 36 

 hours, such as the one given at Columbia 

 University, represents less than one per 

 cent, of the total work required on the 

 4,000 hour basis. 



C. C. Trowbridge. 



Physical Laboratory, 



Columbia University, N. Y. 



SOME PRELIMINARY EXPERIMENTS ON TEE 



MOTION OF IONS IN A VARYING 



MAGNETIC FIELD. 



The experiments described below were 

 suggested by the negative results of V. 

 Cremieu's search for a force acting on a 

 static charge in a varying magnetic field. f 

 The scheme of the Cremieu experiment 

 may be briefly described by the statement 

 that a disc which was charged to a high 

 potential was suspended in the field of a 

 strongly excited electromagnet. Upon 

 breaking the current the disc should have 

 experienced a force in accordance with the 

 Maxwell equation 



1 ;trT 



(1) 



o„,..-lf. 



The quantity of electricity that can be 

 placed on a body of considerable dimen- 

 sions is comparatively small, so, that in the 

 case of the Cremieu experiment e/m was 

 a small quantity. 



It occurred to me to use the negatively 

 charged ions in an ionized gas as the car- 



* ' Education in the United States,' N. M. 

 Butler. 



f Crgmieu, Annates de Chemie et Physique, "th 

 Series, I., 24. 



riers of the static charge; in the ease of 

 ions e/m is very large, being about 4 X 10^^ 

 E.S. An ion, because of its high charge, 

 should move with considerable velocity in 

 a varying field of moderate strength. For 

 the purpose of showing the theoretical mag- 

 nitude of such ionic motion in such a field 

 I will assume an ideal ease. Suppose that 

 a cylindrical vessel is placed in a coil of a 

 few turns through which is passing an oscil- 

 latory current of high frequency. Assume 

 that there is a complete vacuum ex- 

 cept for one negative ion which at the 

 initial time is at rest at a distance r from 

 the center of the coil. The ion will be acted 

 upon by a force the direction of which 

 will be a circle of radius r about the mag- 

 netic center of the coil. Neglecting the 

 centrifugal acceleration and the change in 

 apparent mass due to its motion, if the 

 maximum strength of the field at the posi- 

 tion of the ion is 100 C.G.S. and the fre- 

 quency is 10", it may be shown that the 

 ion would execute a harmonic oscillatory 

 motion in a circular path around the center, 

 with a maximum displacement from the 

 position of rest of 20 cm. and a maximum 

 velocity of 13 X 10^ cm. per second. 



Since for the purpose of experiment, it 

 is desirable to have ions in abundance and 

 a rapidly varying magnetic field, I have 

 made use of the well-known electrodeless 

 discharge in the Tesla oscillatory field, as 

 in this form of discharge the gas is highly 

 ionized and the field is of high frequency. 

 In the actual phenomena the amplitude 

 is of course many times smaller than that 

 calculated above. The ions probably move 

 but a short distance and are then stopped 

 by collisions with the molecules, produ- 

 cing by the collision many other ions which 

 by impact produce yet others and thus the 

 effect accumulates until a strong current, 

 the ring discharge, is produced. 



To demonstrate by experiment that some 

 such motion actually exists I have made 



