PRINCIPLES OF GENERAL PHYSIOLOGY 



where y is the ordinate of any point P measured downwards from the asymptote, 

 that is, the level at which the meniscus finally comes to rest, t is the horizontal 

 distance of P from a point on the asymptote taken as origin of co-ordinates (that 

 is, time from commencement of charge), a and c are constants, e is the base of 

 natural logarithms (see Fig. 203). It will be seen, on reference to the curve, 

 that the potential difference between the terminals can be determined by taking 

 any point on the curve, without the necessity of its having arrived at the limit of 

 its movement ; in fact, it may be brought back at any point in its course by the 

 application of an opposite potential difference, without interfering with the 

 measurement of that which produced the original movement. The potential 

 difference causing any deflection may be considered as made up of two parts, one 

 represented by the ordinate of the curve at any time, and the other represented by 



the vertical distance the 

 meniscus has still to move. 

 The latter is a function of 

 the rate of movement at 

 the time taken, that is, of 

 the steepness of the curve. 

 It is, therefore, measured 

 by the angle made by the 

 geometrical tangent of the 

 curve with the axis of 

 abscissae. The simplest 

 way of analysing a com- 

 plex curve, obtained ex- 

 perimentally, is that of 

 Keith Lucas (1909, 2, p. 

 218). Each tube has, of 

 course, to be calibrated, 

 the rate of movement 

 depending partly on the 

 electrical resistance, partly 

 on mechanical resistance 

 to movement, probably 

 friction. The movement 

 is quite aperiodic. The 

 method of drawing tubes 

 will be found in the paper 

 quoted. 



This electrometer, 

 although sensitive enough 

 for most work, is less so 

 than the string galvano- 

 meter, but, for exact 



analysis, the photographed records of the former present certain advantages in 

 that the analysis is simpler, following a better known law than those of the string 

 galvanometer (see the paper by Keith Lucas, 1909, 2, p. 210). 



Other forms of electrometer have been little used in physiological work, although it seems 

 possible that the striny electrometer, in which the string moves between plates with opposite 

 charges, will be found ; useful (for a description of the instrument, see the Cambridge 

 Instrument CO.'B Catalogue of Electrometers). 



The Circuit. The arrangement of the connections is practically identical with 

 that used in the measurement of the electromotive force of a concentration battery 

 (page 192). The diagram of Fig. 204 may be found useful. 



Rheotomes.The repeating rheotome, by which corresponding bits are cut out, as it were, 

 from a series of electrical responses by means of contacts arranged to be made and broken by 

 a rotating wheel, is rarely used at the present time. The introduction of more accurate and 

 sensitive instruments and means of analysis of photographic curves has practically displaced 

 it. The use of a device for opening a series of keys at known short intervals of time after one 





FKJ. 201. 



DIAGRAM OF MECHANISM OF STRING 

 GALVANOMETER. 



The fine wire (" string "), CC, is stretched in the narrow gap between the 

 poles X and S of a powerful electro-magnet. When a current passes 

 through the "string" in the direction of the vertical arrows, the wire 

 is deflected in the direction of the arrow a, that is, at right angles to 

 the magnetic field AIS. This small movement is observed, or projected 

 on to a photographic plate, by means of a microscope, ED, the light of 

 an arc lamp being condensed by the lens F on to the string. 



