328 Dr. Tyndall on the Progress of the Physical Sciences : 



inversely as their cross sections ; which is a complete verification 

 of the hypothesis of Ohm. 



Experiment 4. — A rectangular wooden trough was con- 

 structed, and its interior was coated with wax. At one end was 

 placed a porous cell containing a solution of sulphate of zinc, in 

 which a plate of zinc was immersed ; the rest of the trough was 

 filled with a solution of sulphate of copper, and at the opposite 

 end a plate of copper was immersed. The zinc and copper 

 plates were connected by a wire. The edge of the trough was 

 graduated ; two copper wires dipped into the solution of sulphate 

 of copper, and by means of the graduation their exact distance 

 asunder could be readily ascertained. One of these wires was 

 well connected with the earth, the other was connected with the 

 upper plate of the condenser. The mode of experiment was, in 

 fact, the same as that pursued with the metallic portion of the 

 circuit. Here also it was found that the tension at the point 

 connected with the discharging wire was zero ; right and left 

 from this point a regular increase of tension was observed ; on 

 that side from which the current proceeded the electricity was 

 positive, on the other side negative. Further, according to the 

 view of Ohm, who imagined the electricity to make its way 

 through the interior of both metallic and fluid conductors, the 

 tension at every point in any given cross section is the same. In 

 the case of a metallic conductor it is, of course, impossible to test 

 this experimentally; but in the fluid portion of the circuit, 

 M. Kohlrausch found exactly the same tension throughout each 

 transverse section, whether he raised or sunk the wire (which in 

 these experiments was everywhere coated with shell-lac except at 

 its extreme end) in the fluid, or pushed it more or less aside 

 laterally*. 



I trust the reader bears in mind what has been said regarding 

 the electric " fall." The greater the resistance offered to the 

 passage of the current, the greater the fall. In a thin wire, the 

 line expressing the tension at every point will be a steeper gra- 

 dient than in a thick wire ; and in the fluid portion of the circuit 

 the gradient may be expected to be steeper than in either of the 

 former cases, for here the resistance is greatest. The simplest 

 possible circuit must therefore exhibit a series of gradients ex- 

 pressive of the tension of its various parts. There is the fall 

 along the connecting wire, the fall along the zinc and copper 

 plates (which, however, is practically zero, as they offer almost 

 no resistance), and the fall along the fluid. But let us suppose 



* Weber and Kirchhoff differ from Ohm here. They do not admit a 

 motion of the fluid through the interior of the conductor, but solely along 

 its surface. Their hypotheses, however, lead them to results which en- 

 tirely agree with Ohm's. 



