OF VITAL PHENOMENA 21 



duetivity vessel, a meter resistance wire with a sliding contact, 

 and a telephone with which to detect the current (Fig. 4). These 

 resistances are connected in a circle. The meter wire is divided 

 by the sliding contact into two resistances which form two legs 

 of the circuit, the box and the cell forming the other two legs. 

 The two secondary poles of the induction coil are connected to 

 the sliding contact and the juncture between coil and box re- 

 spectively. The conductivity cell is marked A and the adjacent 

 leg of the slide wire B, the box marked C and the adjacent leg 

 of the slide wire D. The current entering the circuit at the 

 juncture of A and C divides, part going through A and B and 

 the other part through C and D. If the two ends of the meter 

 wire have the same electrical potential, then the resistances of 



AC A B 



— = — , also — = — . In order to determine whether the two 



B D CD 



ends of the meter wire have the same potential the two wires of 

 the telephone receiver are connected to the two ends and if no 

 sound is heard the two ends have the same potential. If a sound 

 is heard the resistance of the box is changed or the sliding con- 

 tact on the wire is moved until no sound is heard. The resistance 

 of the box may be read in ohms ; the resistances of the two parts 

 of the meter wire are proportional to their length, and hence 

 their lengths are used in the equation. From the equation the 

 resistance of the conductivity cell A may be calculated. The 

 ratio B/D may be read from the accompanying table. 



Usually the telephone is never quite silent, and the point of 

 tone minimum may be more or less sharp, but may be sharpened 

 in the following ways. The electrodes are made larger (their 

 surface area increased). A self inductance, which may be varied, 

 is placed in series with A. An electrical condenser, that may 

 be varied, is placed in parallel with C. By readjusting the in- 

 ductance and condenser and the sliding contact alternately, a 

 sharp tone minimum is finally obtained (Washburn and Bell, 

 1913). The condenser is to compensate for- the capacity of the 

 cell, which is larger the larger and closer together the electrodes 

 are (but is least in the form of cell shown in Fig. 2). The self 

 inductance is to compensate for the self inductance of the box 

 and connecting wires. Resistance coils with very low self in- 



