SCIENCE AND PRACTICE. 



285 



influence of pressure, is strikingly observed in submerging a 

 cable. As the cable reaches the bottom the copper resistance 

 becomes gradually less, showing a lower temperature ; the 

 insulation resistance increases at the same time, owing to the 

 pressure, to a marked extent, and afterwards further increases 

 as the gutta-percha becomes electrically sensible of its altered 

 temperature. 



IV. METHODS OF MEASUREMENT. 



40. Kirchhoff's Laws. Hitherto we have regarded the 

 current as traversing simple closed circuits. Problems often 

 occur in practice in which it is necessary to consider the 

 circuit as made up of several parallel branches, or shunt 

 circuits. The question, for example, whether a single battery 

 could be used for telegraphing at the same time to Bristol, 

 to Hull, and to Paris, belongs to this branch of the subject, 

 as do also the mathematical solutions of the Wheatstone's 

 bridge, PoggendorfFs, and other methods indispensable in 

 electrical measurements. 



Kirchhoff * has provided for the solution of such questions 

 two propositions, which he has proved mathematically and 

 experimentally. 



The first is, that 



" The sum of the intensities in all those wires ivhich meet in a 

 point is equal to nothing." 



o (Fig. 133) is the point in which seven wires meet ; the 

 currents i lt I 2 , and i 3 , ap- 

 proach, and ?!, 2 , %, and 

 i, recede from it. If we 

 give the plus sign to 

 those currents which ap- 

 proach and the negative 

 to those which recede 

 from the point, the sum 

 of all the intensities is, 



* Fogg. Ann. 64, p. 513. 



