chap, xiii.] RESISTANCE OF FLUIDS. 137 



Resistance of fluids. A very simple arrange- 

 ment is shown in Fig. 70 for interposing resistance 

 even to an enormous extent, which could be used for 

 physiological or therapeutical purposes. It consists of 

 a glass tube filled with distilled water and closed at 

 each end by an indiarubber cork. Through each cork 

 is passed a copper wire. If the wires from a battery 

 be connected with the wires from the tube a current 

 will pass in the tube from one wire to 

 the other through the water, and will 



o 



encounter resistance directly proportional 

 to the extent of the layer of water between 

 the two wires. Since the wires may be 

 pushed through the cork so as to ap- 

 proximate to one another, or can be removed 

 still farther in the tube from one another, 

 the resistance can be readily diminished or Fl - g . ro _ Re _ 

 increased. It is calculated that a column of F fu?<L c . e f 

 distilled water, one metre long and one 

 millimetre in diameter, would oppose resistance to an 

 electric current equal to that of a copper wire of the 

 same thickness long enough to make 1G7 turns round 

 the earth. Alcohol added to the water would increase 

 its resistance. The resistance of water could be dimin- 

 ished by dissolving salt in it, or by adding sulphuric 

 acid to it. The objection to water, salt solution, or 

 dilute sulphuric acid, is that by the passage of the 

 current polarisation is set up. As mentioned in the 

 paragraphs on non-polarisable electrodes, solutions 

 of neutral sulphate of zinc are free from this ob- 

 jection. Therefore by taking a narrow tube, say one 

 metre long, graduating it in millimetres, and filling it 

 with solution of sulphate of zinc, of a certain strength, 

 one would have a very simple and, at the same time, 

 useful means of regulating the strength of a current 

 of electricity. 



It should be observed that the amount of resistance 



