Hartmann & Braun, Bockenheim- Frankfort o. M. 



163 



13. Differential Galvanometer with bobbins, wound with 

 resistances in the ratio of 1 : 20. By means of this instrument the 

 resistance of an incandescent carbon filament may easily be found by 

 Voller's method. 



14. Apparatus for determining small resistances according to 

 Kirchhoff's differential shunt method, consisting of a standard resistance 

 of 0.01 ohm, two precision resistance boxes and a differential galvano- 

 meter, the latter modified according to Deprez. 



15. Nippoldt's Testing Apparatus for Lightning Conductors, 

 consisting of Wheatstone bridge reading directly, contained in the casing 

 of a telephone, a small induction co'il together with cells for generating 

 alternating currents, or if desired a simple galvanometer, the whole 

 in a sling case; also a folding earth plate. 



. The resistance due to the passage from the lightning conductor to the 

 earth plates has, on account of the polarization taking place at the plates, like 

 the resistances of electrolytes, to be measured by means of alternating currents, 

 hence a telephone is substituted for a galvanometer. It is useless to measure 

 the resistances of the metallic conductors, as these can only be examined by 

 ocular inspection. 



16. Tangent Galvanometer for currents and E. M. F.'s, 

 with Kohlrausch water voltameter for calibrating by volume of ga(s ge- 

 nerated, and a comparison amperemeter. 



The method of calibration by means of the Kohlrausch water voltameter 

 is extremely simple and does not involve the use of an analytical balance, as is 

 the case with the silver or copper voltameter. The tangent galvanometer has 

 a range of 3 miUiamperes to 25 amperes; with the aid of shunts or additional 

 resistances its range may be extended considerably. 



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