6o A MANUAL OF PHYSIOLOGY 



comparative results can be obtained. It is well, to avoid the risk 

 of accident, to rotate the centrifuge under a guard. 



9. Electrical Conductivity of Blood. (i) Fill a small U-tube with 

 blood up to a mark. In each limb insert a platinum electrode* 

 connected with a holder, which insures that the electrode shall 

 always dip to the same depth into the tube. Arrange the U-tube 

 so that it is immersed at least to the mark in water of constant 

 temperature. Water running freely from the cold-water tap into 

 and out of a large vessel will have a sufficiently constant temperature 

 for the purpose. A thermometer must be fixed in the water with 

 its bulb in contact with the U-tube. Connect the electrodes with a 

 resistance-box in the Wheatstone's bridge arrangement (Fig. 204, 

 p. 617), so that the U-tube occupies the position of the unknown 

 resistance CD. Instead of the battery F, connect the poles of the 

 secondary of a small induction-coil, arranged for an interrupted 

 current, with A and C, and instead of the galvanometer G insert a 

 telephone. The resistances AB and AD (the arms of the bridge) 

 will be obtained by taking out two plugs from the appropriate part 

 of the resistance-box. Whether the arms should be equal (say, 

 10 : 10, 100 : 100, or 1,000 : 1,000 ohms) or unequal (say, 10 : 100, 

 or 100 : 1,000, or 10 : 1,000 ohms) will depend upon the resistance 

 of the tube of liquid to be measured. Take out from the part of the 

 box corresponding to BC a plug representing a resistance some- 

 thing like that which the tube of blood is expected to have. Close 

 the primary circuit of the induction-coil, and apply the telephone to 

 the ear. A buzzing sound will be heard, which will be louder the 

 farther from the true resistance of the tube the resistance taken 

 out of the box is. Go on altering the resistance in the box by taking 

 out or putting in plugs till the sound disappears, or is reduced to a 

 minimum. The temperature of the water should now be read off. 

 The resistance of the tube of blood for this temperature can easily 

 be calculated from the formula on p. 617. It increases about 

 2 per cent, for each degree Centigrade of diminution of temperature. 

 The conductivity is the reciprocal of the resistance. By determining 

 once for all the resistance of the tube when filled with a standard 

 solution of a salt whose conductivity is known, the specific conduc- 

 tivity of the blood can be expressed in definite units, but this is not 

 necessary for the purposes of the student. Compare the resistances 

 of defibrinated blood, serum, o~g per cent, sodium chloride solution, 

 and a sediment of blood-corpuscles separated by centrifugalization. 



(2) Instead of the resistance-box a wire mounted on a scale may 

 be used for the bridge arms AB, AD, the ends of the wire being 

 connected at B and D. A slider with an insulated handle moving 

 along the graduated wire is joined by a flexible wire with one pole 

 of the secondary coil, the other pole being connected at C. The 

 resistance BC is constituted by a rheostat from which a fixed resist- 

 ance can be taken out. Instead of obtaining the minimum sound in 

 the telephone by varying the resistance BC in the box, the measure- 

 ment is made by varying the position of the slider ; in other words, 

 by changing the ratio AB : AD. 



(3) If no rheostat is available instructive comparative measure- 

 ments may still be made with the graduated wire by substituting 

 for the resistance BC a U-tube of another liquid. 



* If the platinum electrodes are of good size and the resistance of the tube 

 of liquid considerable, it is not necessary to platinize them i.e., to cover 

 them by electrolysis of a solution of platinic chloride with a layer of 

 platinum-black. 



