Mr. C. Tomlinson on the Weathering of Rocks. 475 



series-machine. If short-circuited it will require an infinite 

 speed to become self-exciting. 



There is a still more general way of establishing the rela- 

 tion (6). Every dynamo may be regarded as an arrangement 

 of two circuits, a primary (or field-magnet) circuit and a 

 secondary (or induced or armature) circuit, between which 

 there is a coefficient of mutual induction M, which is caused 

 to vary by mechanically altering the configuration of the 

 system. If the currents in the two circuits be called respect- 

 ively /\ and i 2 , then the work done in starting or stopping 

 either current in the presence of the other is equal to M?'^. 

 Now in each revolution of the armature of an ordinary dynamo 

 the current in each of the sections of the armature-coil is 

 twice started and twice stopped. Hence in each semi- 

 revolution the work to be mechanically performed in driving- 

 is equal to M/j/g ; or if, as in the series-dynamo, the current 

 in the two circuits is the same, the work to be done is Mi 2 . 

 Now, on the assumption that there is no magnetic leakage, 

 the value of M is equal to 47tS 1 S 2 h-2/9 ; where S], and S 3 are 

 the respective numbers of convolutions of the primary and 

 secondary coils which surround the tubes of magnetic force 

 in the magnetic circuit. For our purpose, S 2 , in ordinary 

 armatures, with symmetrically-wound coils united in a self- 

 closed circuit, tapped at two opposite points by the two con- 

 tact-brushes, and with commutation occurring at points where 

 each commuted convolution encloses a maximum magnetic 

 flux, is equal to JC. But the work done by the machine in 

 the electric circuit during a semi-revolution is equal to 

 r1Il-r-'2n. Equating this to the work done on the machine 

 gives, as before, 



City and Guilds Technical College, 

 Finsbury, June 1888. 



LV. Remarks on the Weathering of Rocks, and certain Elec- 

 trical Phenomena, suggested by some Statements in a popular 

 Novel. By Charles Tomlinson, F.R.S. fyc* 



THE demand for works of fiction has been increasing of 

 late years at so considerable a rate that the chief business 

 of a large circulating library consists in dealing out novels to 

 abscribers. A cursory examination of the contents of a 

 railway bookstall shows by the gaudy covers of its books that 

 cheap reprints of popular novels form the staple commodity. 

 * Communicated by the Author. 



