258 HOWARD J. CURTIS 



about drift in a galvanometer. On the other hand, for hieh resistance 

 sources a vacuum tube amplifier has a tremendous advantage over 

 the galvanometer even for d.c. measurements. 



6. Power Supplies 



Batteries are certainly the easiest satisfactory means of supplying 

 power for an amplifier. However, they are expensive, awkward, and 

 are continually running down, so that battery-operated amplifiers 

 require constant attention. The usual type of power supply is not 

 stable enough for most work, but electronically regulated power sup- 

 plies have proved to be eminently satisfactory. One such power 

 supply is shown in Figure 9 (p. 257). Here the four power tubes (6L6) 

 in parallel act as a variable resistance in series with the filtered power 

 supply. Any changes that take place in the output voltage are 

 amplified and applied to the grids of these power tubes to change their 

 effective resistance by the amount necessary to bring the output po- 

 tential back to normal. This power supply will deliver up to 450 ma. 

 at 300 V. 



The author has recently built a high gain d.c. amplifier in which a 

 single regulated power supply furnished current for both plates and 

 filaments of all tubes. The tubes had filaments requiring 12 v. and 

 0.15 amp., and all filaments were run in series. Operation was as 

 stable as if the first stages were run entirely on new batteries. 



7. Recording Equipment 



A wide variety of different instruments has been developed for 

 recording bioelectric potentials. Most of the present day instru- 

 ments were originally developed either for the purpose of obtaining 

 an instrument with a faster response, or a greater sensitivity than was 

 previously possible. Since vacuum tube amplifiers are now so satis- 

 factory, sensitivity is no longer a major consideration except in special 

 cases. So speed of response and convenience have become the major 

 factors in the choice of instruments. 



For either visual observation or photographic recording w^here 

 speed of response is a consideration, the instrument of choice is a 

 cathode ray oscillograph. For practical purposes the instrument has 

 no inertia whatever and recordings thus represent a true pattern of 

 the potential changes applied to the instrument. There are a num- 

 ber of very satisfactory instruments on the market, complete with all 



