400 Mr. H. Tomlinson's Note 



crosspiece of wood M ; M, and are insulated from the tongues 

 by india-rubber. 



A quadruple crank, Q, is arranged to be rotated between 

 the supports of the crosspiece M, M by means of the handle H. 

 The crank is put under the tongues at the points where they 

 are pressed by the springs. By rotating this crank four 

 alternate plungers (I, III, V, VII) are lifted out of their 

 mercury-cups, the other four (II, IV, VI, VIII) being pressed 

 in by the springs; or, on turning the crank through 180°, 

 the four tongues (II, IV, VI, VIII) are lifted out and the 

 other four pressed into contact by their springs. 



The electrodes of the resistances to be compared are intro- 

 duced into the mercury-cups a, /3 and 7, S respectively, or 

 they could be soldered to continuations of the tongues with 

 which these cups are connected. 



Thus rotating the crank determines one of the following 

 series of connexions : — 



1st position. Contacts II. 3 III. 5 V. 7 VII. 



Connexions Aa. B /3. C 7. D S. 



2nd position. Contacts 2 II. 4 IV. 6 VI. 8 VIII. 



Connexions AS. B 7. C /3. Da. 



So that the coils X and Y are alternately connected with A B 

 and C D, and vice versa, by a half-rotation of the crank. 



It is necessary that any method of interchanging the coils 

 should not alter the resistance of the connexions. It will be 

 seen that the arrangement described is quite symmetrical, and 

 therefore satisfactory in this respect, except for the piece of 

 copper connecting I, II with 8 ; and the resistance of this, if 

 the copper be fairly thick, is too small to be appreciable. 



Cavendish Laboratory, Cambridge, 

 March 7, 1884. 



XLV. Note on Hall's Phenomenon. 

 By Herbert Tomlinson, B.A* 



IN a paper entitled " The Influence of Stress and Strain on 

 the Action of Physical Forces " t, I drew attention to the 

 fact that there is a marked resemblance between the table of 

 " rotational coefficients" drawn up Prof. Hall and that laid 

 down by myself from the results of experiments on the effects 



* Communicated by the Physical Society. Eead March 22, 1884. 

 t Phil. Trans. Part I. 1883, p. 168. 



