1891.] Presents. 257 



currents, I have calculated in 1 expressions for the intensities of the 

 force and couple respectively. 



Another experiment is the following : A sheet of copper is placed 

 so as to half cover an alternating magnetic pole. Upon this, near the 

 pole, is laid a hollow sphere of copper. The electromagnetic action 

 produces a couple so powerful that the friction of rotation is over- 

 come, and the sphere spun round. 



In order to throw light on this, after a theorem in 2 as to the 

 kind of currents set up in a conductor, I have considered a number 

 of cases. A thin circular infinite cylindrical shell lies in an alter- 

 nating field of currents parallel to its axis, and the couple upon it is 

 found. The result is applied to give the couples on two such shells 

 in the presence of a parallel current and of a pair of currents forming 

 an electromagnet. 



The couple in action upon a thin spherical shell in a general 

 periodic field, has next been found, and is applied to give the couples 

 on two thin shells under the influence of 



(i.) An alternating current in a straight infinite wire. 



(ii.) A pair of such currents forming an electromagnet, 

 (iii.) An alternating magnetic pole, 

 (iv.) An alternating electromagnet of very short length. 



It transpires that, whatever the field in action upon the cylinder and 

 sphere (the former consisting of currents parallel to the axis), there 

 will be no couple if the field be completely in one phase. Thus, the 

 sphere of Professor Thomson is made to spin because the currents 

 induced "in the copper plate do not coincide in phase with those of the 

 magnet, and not (as has been stated) because it acts as a screen and 

 renders the field unsymmetrical. Were the plate a perfect conductor, 

 it would be a perfect screen, but there would be no couple. 



Presents, December 10, 1891. 

 Transactions. 



American Association for the Advancement of Science. Pro- 

 ceedings. 1890. 8vo. Salem 1891. The Association. 

 Athens: National Library. KaraAofyos -rCov Ei(3\iwv -riy? 'E0viici]<; 

 Bi(B\ioOrjKfj<? T^? 'EXXao9. Tytd^aa 7'. AanviKrj $4X0X07/0. 4to. 



Athens 1890. The Library. 



Baltimore : Johns Hopkins University. Circulars. Vol. X. 



Nos. 89-91. 4to. Baltimore 1891 ; Studies (Historical and 



Political Science). Series 9. Nos. 7-10. 8vo. Baltimore 1891 ; 



Theses. 1890-91. 8vo. Baltimore. The University. 



Peabody Institute. Annual Report, 1891. 8vo. Baltimore. 



The Institute. 



