44 



DESIGN PROCEDURE 



The procedure to be described in this chapter is only one of many possible. 

 It is the way the author works, but he anticipates that readers will soon 

 develop their own methods of attack. The steps which lead from the original 

 conception of a new piece of apparatus to its final realization in the metal 

 are illustrated by the use of an example, the production of a complete 

 electrophysiological unit for students' use. 



We begin with a specification, which may run something like this: 

 Stimulator — two repetition rates, ISJSQC and 1/sec; two shocks to be 

 provided, each with a delay variable up to 30 msecs from the initiation of the 

 C.R.T. time-base sweep. Exact shock waveforms immaterial, but shock- 

 strength adjustable up to a maximum equivalent to a rectangular pulse 

 of 30 V amplitude into 10 kQ and lasting \ msec. 



Time base — triggered by stimulator, providing trace lengths adjustable 

 from 10 to 80 msec. Switchable 10 msec time-marking pips to be provided 

 so that the trace length may be set up to an exact multiple of 10 msec within 

 the range provided. Pips derived from the 50 cycle mains would be sufficiently 

 accurate. 



Shock No.l. 

 . out to 

 preparation 



Figure 44.1 



Amplifier — A direct-coupled amplifier of maximum gain such that the 

 cathode ray tube spot is deflected to top or bottom of the screen by ±25 mV: 

 this is for intracellular work. In addition, a pre-amplifier, a.c. coupled, of 

 short time constant, gain 100 times, suitable for the detection of weak action 

 potentials. 



Presentation — 6 in. diameter flat-faced cathode ray tube, medium per- 

 sistence (i.e. green) phosphor. 



Power supplies — As requisite. 



671 



