ELECTRICAL CHANGES IN TISSUES 



641 



on it, which kept it in its place in the tube, further towards the point. If 

 negative, the surface tension is increased and meniscus retreats, since its surface 

 tension, which was previously balanced by a given external pressure, will now 

 only be balanced by this same pressure at a wider part of the tube. Until the 

 mercury has come to rest, a certain charge of electricity flows into the instrument, 

 which behaves as a condenser. After this no current flows. The movement may 

 be very rapid and is completely aperiodic. The contact surfaces of the acid and 

 mercury are polarised, but the theory is somewhat complex, and may be found in 

 Freundlich's book (1909, pp. 184-212). Roughly speaking, it may be said that 

 when the mercury meniscus is positive it becomes oxidised and dirty, when 



1164 



FIG. 200. General appearance of the string galvanometer, as made by the Cambridge Scientific 

 Instrument Co., in accordance with the pattern designed by Einthoven. 



A, 'Poles of electro-magnet, itself excited by current through coils BB. 



C, The fibre case, containing the stretched "string." 



G, D, Microscope and condenser, passing through holes in the pole pieces. 



E, Narrow air gap between poles, containing the string. 



J, Adjustment for tension of string. 



ft, Screw for centring the string case, which rocks on the two screws K. 



R, Fine adjustment of microscope. 



(Cambridge Sci. Instr. Co., Catalogue No. 126, p. 23.) 



negative it is reduced to bright metal. Hence the surface tension is low in the 

 former case, high in the latter. The movements are photographed by projection 

 on a slit, behind which a photographic plate is moved. The curves obtained 

 require correction, but the law governing the position of the meniscus at any 

 time after connection with a source of electromotive force is a definite one and 

 readily determined for each instrument (see the papers by Burch, 1890, 1892, 

 and by Keith Lucas, 1909, 2). The curve is a logarithmic or exponential one, 

 with the equation : 



or, 



