INTRODUCTION 



rather than to time. For brevity we shall also call Figure 1.2 the 'constant- 

 voltage alternator'. 



(3) The direct current, constant-current generator (Figure 1.3). This is 

 supposed to force a constant direct current / through the load, whatever the 

 nature of the load. 



(4) The alternating current, constant-current generator (Figure 1.4). This 

 forces a current / = / sin oit through the load, whatever the nature of the 

 load. We shall also call this the 'constant-current alternator'. 



Notice that the direct voltage and current generators are drawn with a 

 switch. The former is 'on' when the switch is closed, and the latter is 'on' 

 when the switch is open. The reason for the presence of these switches is 

 important, and necessitates a rather long digression. 



Voltage 



or 

 current 



Figure 1.3 Figure 1.4 



Electrotechnology may be divided into two kinds, 'Power' engineering and 

 'Control' engineering. In the former the commodity is electrical energy, 

 and the object is to convey this in a continuous and satisfactory manner from 

 the source to the consumer: the abiding consideration is efficiency. In 

 control engineering the commodity is change, information, a message or 

 signal. Power efficiency is usually unsought here ; the important thing is to 

 minimize any distortions which the signal may undergo in its passage through 

 the apparatus. 



Electronics in biological research is a department of control engineering. 

 The techniques allow information about biological happenings to be trans- 

 ferred to the observer, and there is a sense in which they allow signals to be 

 passed from the observer to the biological system, e.g. as in deUvering a 

 controlled stimulation. It follows that in descriptions of the performance of 

 such apparatus, the amount of distortion which it introduces recurs fre- 

 quently. 



Distortion may be placed upon a quantitative basis by considering the 

 difference between the ideal response and the actual response obtained, but 

 this cannot be stated for every possible kind of signal, since signals are 

 usually innumerable. The best that can be done is to consider the distortions 

 arising when apparatus receives special signals of a simple and artificial 

 sort. 



In electrobiology, signals are mainly of the continuously variable kind, 

 that is, the apparatus is analogic rather than digital; a voltage or current 

 represents the extension of a muscle, the light reflected by a retina, or a 

 membrane potential. Our test signals, which will allow us to estimate the 

 performance of our apparatus, will therefore be continuous, single-valued 

 functions of time. In practice, the two commonest are: (1) the step-function 

 (Figure 1.5) in which the input to the apparatus changes abruptly from one 



4 



