294 THE ELECTRIC TELEGRAPH. 



of galvanic circuits are found of great value, frequently 

 saving time in developments. 

 These corollaries are : 



1. If, in any system of circuits, containing any electro- 

 motive forces, a conductor exists in which the current = 0, 

 the currents in the remaining circuits are not altered in the 

 least degree if the circuit of the conductor in question is 

 divided, or it is removed, together with whatever electro- 

 motive force it may contain, from the system. 



2. If the conductor in question contains no electro-motive 

 force, the currents will not be altered, if, after its removal 

 the points between which it previously existed are connected 

 directly with each other. If, on the other hand, it contain 

 an electro-motive force, the points can only be joined again 

 by inserting between them an equivalent electro-motive 

 force. 



3. In a system of linear conductors containing electro- 

 motive forces, the current set up in any conductor, a, by an 

 electro-motive force contained in any other conductor, b, will 

 be identically the same as that which would be set up in b 

 by an equal electro-motive force in a. 



4. If in a svstem of linear conductors there are two of 



/ 



them, a and >, in which the electro-motive force in a occasions 

 no current in b, whatever current may be circulating in b will 

 not be altered if a is divided or removed, nor will the current 

 in a be altered if b is divided or removed, however the electro- 

 motive forces in the remaining circuits may be arranged. 



42. Siemens's Apparatus for Testing Cab/es. The testing 

 apparatus, as at present used in the testing- room of 

 Messrs. Siemens, is a modification of their old plan. It 

 was rearranged and endowed with its present form in order 

 to render it available, not only for measurement by the 

 bridge method, but also for measurements of great resistance 

 by deflection of the galvanometer needle, of sensibility of the 

 instrument, of the electro-motive force of the battery, and of 

 charge and discharge of a cable. 



For the construction of such an apparatus, the following 

 parts are necessary : An adjustable resistance coil of 1 to 



