382 



TENTH ANNUAL REPORT OF 



and to restore it to its original position, the resistance of the "battery- 

 is diminished by aid of the rheostat and resistance rolls,* nntil the 

 needle comes to rest at its first position. The reduced length of the 

 wire tlius brought from the battery is the measure of the resistance of 

 one inch of liquid ; the influence of polarization has already been 

 eliminated by the method of the ex])eriment. 



The arrangements which Horsford and Becquerel used, to measure 

 the resistance of liquids, are founded on the same principle. 



Horsford used a quadrangular trough of solid wood (Pog. Ann. 

 LXX, 238) 3 decimetres in length and 7| centimetres in breadth and 

 Fig. 25. depth, for holding the liquid, 



(Fig. 25 ;) the inside is coated 

 with shellac varnish to prevent 

 the escape of the liquid. Two 

 pieces of wood are placed on the 

 trough ; one of which. A, is fas- 

 tened, while the other, B, can 

 be moved back and forth as a 

 slide. These cross-pieces serve 

 for holding the immersed plates 

 in the liquid, and for changing their distance apart at pleasure. The 

 plates, the same width as the trough, are fastened to copper strips, 

 which are again screwed to the cross-pieces. 



The trough, filled with the liquid, is now placed with the rheostat 

 and tangent compass in the closing arc of a battery of more or less 

 cups, according as the circumstances require a greater or less electro- 

 motive force. The course of the experiment is similar to that indi- 

 cated by Wheatstone. 



Horsford's arrangement has many advantages. 1. The measure- 

 ments can be extended by placing the plates at a greater number of 

 distances apart ; 2. Plates of different metals can be easily substitu- 

 ted ; and 3. Experiments can be made with the trough filled to dif- 

 ferent heights. 



Horsford has shown that liquid columns follow exactly the same 

 law in regard to resistance as metallic wires ; that is, the resistance 

 is directly as the length, and inversely as the section of the liquid 

 stratum. 



The trough being filled with dilute sulphuric acid, the plates were 

 placed 2.5 centimetres apart, and the entire resistance so regulated 

 that the needle of the compass {5ame to rest at a given point, (say 20°.) 

 The second column of the following table indicates the number of 

 rheostat coils (of German silver wire) which were removed from the 

 circuit to restore the compass needle to the same place, when the dis- 

 tance apart of the plates (the trough being kept filled to the same 



* If the requisite changes of resistance exceed the limits of the rheostat, the object is 

 accomplished l)y the insertion or removal of wire rolls (thin wire wound between the fine 

 screw-thread of a dry wooden cylinder) the resistance of which is known. By adding or 

 taking away such resistance rolls the greater changes of resistance are accompl itched, and 

 the smaller ones are produced by the rheostat alone. 



