.s/A' WILLIAM SIEMENS, F.R.S. 121 



produce the same phenomenon as before. Practically it is not 

 even necessary to give any external impulse upon restarting the 

 machine, the residuary magnetism of the electro-magnetic arrange- 

 ments employed being found sufficient for that purpose. 



The mechanical arrangement best suited for the production of 

 these currents is that originally proposed by Dr. Werner Siemens 

 in 1857,* consisting of a cylindrical keeper hollowed at two sides 

 for the reception of insulated wire wound longitudinally, which is 

 made to rotate between the poles of a series of permanent magnets, 

 which latter are at present replaced by electro-magnets. On im- 

 parting rotation to the armature of such an arrangement, the 

 mechanical resistance is found to increase rapidly, to such an 

 extent that either the driving-strap commences to slip or the 

 insulated wires constituting the coils are heated to the extent of 

 igniting their insulating silk covering. 



It is thus possible to produce mechanically the most powerful 

 electrical or calorific effects without the aid of steel magnets, which 

 latter are open to the practical objection of losing their permanent 

 magnetism in use. 



ON A RESISTANCE-MEASURER. 

 BY C. W. SIEMENS, F.R.S.f 



FOR the measurement of small resistances the method formerly 

 employed was that of the tangent galvanometer, which method is 

 still valuable in the determination of resistances which are in- 

 separable from a difference of electrical potential, such, for instance, 

 as a galvanic element. 



In measuring wire-resistance, more accurate and convenient 

 methods have been devised, amongst which that of the common 

 differential galvanometer and that known as Wheatstone's balance 

 hold the most prominent places. 



But both these systems have disadvantages which render them 

 insufficient in a great many cases. For instance, in the first 



* See Du Moncel "Sur 1'Electrioite," 1862, page 248. 



t Excerpt Philosophical Magazine, Vol. 34, 1867, pp. 270-273. Communicated 

 through the Electrical Standards Committee. 



