PRACTICAL EXERCISES 69 



nected 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 ther- 

 mometer must be fixed in the water with its bulb in contact with the 

 U-tube. Connect the electrodes with a resistance -box in the Wheat- 

 stone's bridge arrangement (Fig. 231, p. 726), 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 will 

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

 resistance-box. Whether AB and AD 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 something 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 tempera- 

 ture 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. 726. 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 conductivity 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, 0*9 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 resistances 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 consti- 

 tuted by a rheostat from which a fixed resistance can be taken out. 

 Instead of obtaining the minimum sound in the telephone by varying 

 the resistance BC in the box, the measurement is made by varying the 

 position of the slider; iu other words, by changing the ratio AB: AD. 



(3) If no rheostat is available instructive comparative measurements 

 may still be made with the graduated wire by substituting for the 

 resistance BC a U-tube of another liquid. 



If the tubes are of the same dimensions, and the liquids with which 

 they are filled are approximately at the same initial temperature, it 

 is not necessary to immerse them in water at constant temperature. It 

 is sufficient to place them side by side in the air. Perform the following 

 experiments in this way : 



(a) Label the tubes A and B. Fill them both to the mark with 

 0-9 per cent. NaCl solution. Connect as in the figure, and move the 

 slider along the wire till the sound is a minimum. Probably the two 

 tubes are not exactly of the same dimensions, and therefore the slider 

 will not be exactly in the middle of the wire. Suppose it is at 49-0 



