ELECTRICAL CHANGES IN TISSUES 



639 



we obtain what is 

 known as the "figure 

 of merit " (E). 



E = 



or, - 



100 xD 



T 2 (R)* ' 



100 x D, x R 



slower rate of movement, so that, as a rule, some compromise has to be made. In 

 -any case, the damping should not exceed that necessary for aperiodic movement. 

 To illustrate the point, Figs. 197 and 198 may be consulted. It will be seen 

 that the deflection is aperiodic in both cases, but with a slight tension on the 

 string the deflection is as large with a small current as that with a larger current 

 if the string is tighter. But the rate of movement, or time taken to reach the 

 final position, is 

 more rapid with the 

 tighter string. If 

 rapid changes are to 

 be followed cor- 

 rectly, therefore, the 

 string must be tight 

 enough to move as 

 quickly as the elec- 

 trical effect to be 

 observed. Fig. 199 

 (from the book by 

 T. Lewis, 1913) 

 shows the different 

 rate of movement of 

 the string under 

 different tensions. 

 If too slack, it does 

 not follow quick 

 changes, like the 

 first ventricular 

 phase of the heart, 

 with sufficient 

 rapidity to give 

 their full value. 



Figure of Merit. 

 The sensitivity 

 may be considered 

 to be the deflection 

 produced by a given 

 current. To com- 

 pare different gal- 

 vanometers, the 

 period of vibration 

 -and also the resist- 

 ance should be taken 

 into account, so that 



II 



g 



"o ^ 



!.^1 



""is o 

 1 1 : 



o 



a 



|J 



where T is the periodic time in seconds, 



R is the resistance of the galvanometer in ohms, 



D is the deflection in mm. for one microampere at one metre, 



Dj is the deflection in mm. for one microvolt at one metre. 



For the properties to be taken into account in choosing a galvanometer for 

 a particular purpose, the catalogue of the Cambridge Scientific Instrument 



