REFERENCES 



instantaneous signal input, and as the back e.m.f. is proportional to 

 d^/d? we have on recording 



and on replaying 



No compensation is necessary. The authors reported satisfactory repro- 

 duction of 1 msec square waves in this manner. The method is not suitable 

 for low frequencies, however, for if djyjdt = Ke 



then cl> = K . ^ edt 



When </) reaches tape saturation point the system breaks down and there is 

 therefore a limit to the voltage-time integral for a pulse. Therefore long 

 pulses can only be small. 



For recording signals down to zero frequency a system has been described 

 by Daniel^ in which constant-current recording is used and on playback 

 a special head is employed which measures ^ instead of d^/d/. The manu- 

 facture of the necessary head should be within the capabilities of most 

 laboratory workshops, but the author has no experience of the technique 

 and cannot say whether this is in fact so. Recording down to zero frequency 

 can be performed using frequency modulation and a conventional tape 

 deck ; since only a little extra electronic circuitry is required this is probably 

 the simpler expedient. An early FM tape recorder was described by 

 Molyneux^ and operated in the range 0-100 cycles, satisfactory for cardio- 

 graphic recording. The method was later extended by the author^" to 

 work with input signals up to 2 kc/s. This made possible the recording of 

 slow electrophysiological phenomena and, for most purposes, a passable 

 reproduction of spike potentials. 



REFERENCES 



^ Personal communication, 1957 



2 Levenson, G. I. P. ./. sci. Instrum. 27 (1950) 170 



3 HoLDEN, A. v., Levenson, G. I. P. and Wells, N. J. sci. Instrum. 28 (1951) 318 

 ^ Daniel, E. D. and Axon, P. E. J. Instn. elect. Engrs. Pt. Ill, 100 (1953) 157 



5 Carter, J. M. Wireless World 59 (1953) 108 

 « Leithead, L. a. and Thompson, L. C. J. Physiol. 126 (1954) 2P 

 ' Coaton, J. W. and Whitfield, I. C. J. Physiol. 125 (1954) 13P 

 8 Daniel, E. D. Proc. Instn. elect. Engrs. Pt. B, 102 (1955) 442 

 ^ MoLYNEUX, L. Electron. Engng. 24 (1952) 130 

 1" Donaldson, P. E. K. Electron. Engng. 27 (1955) 543 



642 



