APPARATUS 



35 



available, and as a result improved methods are now possible. Reference may be 

 made to Harrison (1930), Morton (1931), Elema (1932) and others. 



In fig. 8 is shown the circuit of a convenient instrument, incorporating an 

 electrometer valve, which is constructed by the Cambridge Instrument Company. 

 By simple adjustment the instrument may be used for the direct measurement of 

 potentials or of pH. 



Further consideration is given to the choice of instruments in the next chapter. 



Arrangement of Apparatus 



In studying bacterial cultures the cells (1-8) containing the cultures (the author 

 finds eight cultures a convenient number for simultaneous investigation), the KCl 

 reservoir and the standard cell (S) are placed in the incubator (I) and leads from 

 these run to a series of single-pole double-throw knife switches which enable each 



7 8 



/ 



H^'~~^ 



^^ Earth 



^ 



Fig. 9 



Eh of bacterial cultures 



Diagram of circuit 



culture-cell to be switched into circuit in turn. These single knife switches are 

 outside the incubator and the centre terminals are joined to a double-pole double- 

 throw switch which enables the connections to the potentiometer (P) to be reversed. 

 The arrangement is represented in fig. 9. These simple switching devices serve 

 to avoid the risk of earthing and polarising the electrodes involved in touching bare 

 leads. The lead from the standard half-cell runs to the electrometer needle E. The 

 completed arrangement therefore, beginning in the reverse order, is as follows : — 

 electrometer needle, standard half-cell, agar bridge, KCl reservoir, agar bridge, culture, 

 platinum electrode, single-pole switch, double-pole switch, potentiometer, joined 



