PHYSIOLOGY 



CHAP. 



substituted for sodium chloride. A suitable adaptation of these to physio- 

 logical purposes is the model of Oker Blom (1900). Two glass tubes are 

 sealed at the bottom in the flame, with a little mercury on the base, by which 

 contact is made with two platinum wires that pass through the sealed ends. 

 Pure calomel is placed on the mercury, and above that physiological salt 

 solution, which is brought into contact 'with the [muscle by a tag of cotton 

 saturated with the solution (Fig. 46). 



The galvanometer can be replaced by Lippmann's capillary electrometer, 

 which has the advantage of reacting to very rapid oscillations of current, with 



FIG. 47. Lippmann's capillary electrometer. A, viewed as a whole (pressure bulb, capillary, and 

 microscope); 11, tube (Hg) and capillary (c) which dips into the tube of sulphuric acid 

 (H 2 SO.4) ; C, mercury in capillary tube under the microscope. 



no lost time and no periodic vibrations. Moreover, as the resistance in the 

 capillary is enormous and the current passing through it is practically abolished, 

 unpolarisable electrodes can be dispensed with. As seen in Fig. 47, the 

 instrument consists of a glass tube drawn out in the flame at one end to a 

 capillary 20-30 mm. diameter. This tube is filled with mercury and joined to 

 an apparatus by which the pressure can be regulated. The open end of the 

 capillary dips into 10 per cent sulphuric acid solution. Two platinum wires con- 

 nect the mercury and sulphuric acid, respectively, to the points of the organ under 

 investigation. Under the microscope the excursions of the mercury meniscus 

 which is brought into the field by means of the pressure apparatus can be 

 seen plainly on closure of the circuit. The meniscus advances or recedes towards 

 the end of the capillary according as the potential rises or falls on the side of 

 the mercury tube, and vice versa as regards the reservoir of sulphuric acid. 

 In the capillary electrometer the excursions of the meniscus do not 



