1873.] Louis Schwendler — On Differential Galvanometers. 9 



Applications. — Though the problem in its generahty has now been 

 entirely solved, it will not perhaps be considered irrelevant to add here some 

 applications. 



For our purpose differential galvanometers may be conveniently divided 

 into two classes, viz., those in which the resistances to be measured vary 

 within narrow limits, and those where these limits are extremely wide. 



To the first class belong the differential galvanometers which are used 

 for indicating temperature by the variation of the resistance of a metallic 

 wire, exposed to the temperature to be measured. As for instance, C. W. 

 Siemen's Eesistance Thermometer for measuring comparatively low tempera- 

 tures, or his Electric Pyrometer for measuring the high temperatui'e in 

 furnaces. 



It is clear that for such instruments the law of maximum sensitive- 

 ness should best be fulfilled for the average resistance to be measured, which 

 average resistance under given circumstances is always known. 



To the second class belong those differential galvanometers which are 

 used for testing Telegraph lines, at present the most important application 

 of these instruments. In this case each differential coil should consist of 

 separate coils connected with a commutator in such a manner that it is 

 convenient to alter the resistance of each coil according to circumstances, 

 i. e., connecting all the separate coils in each differential coil parallel, when 

 the resistances to be measured are comparatively low, and all the separate 

 coils consecutively, if the resistances to be measured are high, &c., &c., ful- 

 filling in each case the law of maximum sensitiveness for certain resistances, 

 which are to be determined under different circumstances differently, but 

 always bearing in mind that it is more desirable to fulfil the law of maxi- 

 mum sensitiveness for high resistances, when the testing current in itself is 

 obviously weak, than for the low resistances. 



An example will shew this clearer. Say for instance a differential 

 galvanometer has to be constructed for measuring resistances between 1 and 



10,000. A Siemen's comparison box of the usual kind f ■) being at 



disposal, it mil be convenient and practical to decide that the two differen- 

 tial coils should be of equal magnetic momentum, from which it follows that 

 C as well as p must be unity, or in other words that the two coils must be 

 of equal size, shape and distance from the needle, and must also have equal 

 resistances, i. e., must be filled with copper wire of the same diameter. The 

 resistance of each coil is then found by 



where / is the resistance of the battery and iv a certain value between 

 2 



