VIII. BIOELECTRIC MEASUREMENTS 253 



pletely iinnece.ssary. Tlie principle of the balanced amplifier witli 

 common mode degeneration can be illustrated best by means of a 

 diagram such as shown in Figure 7. It will be seen that the cathode 

 current of both tubes flows through the resistor Re causing a potential 

 drop that tends to make the cathode positive with respect to ground. 

 The value of Re should be such that the potential drop across it is 

 greater than the value of Ei by the desired grid voltage. This means 

 that the grids will be at ground potential. The variable resistance, 

 Rd, should be very low compared to Re and is used merely for Ijalanc- 

 ing the two tubes. Let us assume that a potential is applied across 

 the input terminals such that the grid of Ti is made negative with 

 respect to the grid of T^. Then the current through Ti will decrease 

 and that through T2 will increase by exactly the same amount. The 

 current through Re is unchanged and therefore the potential of the 

 cathodes will remain unchanged. The potential across the output 

 will be an amplified version of the input potential. However, sup- 

 pose now that the potential of both input terminals is raised relative 

 to ground. The current through both tubes will tend to increase. 

 This will tend to increase the current through Re and consequently 

 the potential drop across Re and thus raise the potential of the cath- 

 odes. If Re is large enough, the cathodes will increase practically as 

 much as the grids, leaving the effective grid potential practically un- 

 changed. This would mean that the plate current would remain 

 almost unchanged and the output would remain unaffected. Thus a 

 signal common to both grids (common mode) is degenerated prac- 

 ticall}'' to zero, while signals applied between the two grids (differen- 

 tial signal) is amplified. 



Set-ups can always be arranged so tliat stray induced currents 

 are applied common mode but the potential in question is applied dif- 

 ferentially. With careful adjustment the ratio between the effect 

 produced by a common mode and a differential signal can be made 

 as high as 25,000 to 1. However, a ratio of a few thousand to one is 

 usually ample for most situations. This simple principle of common 

 mode degeneration is of inestimable value in measuring bioelectric 

 potentials. 



One of the diffi(;ull features of direct current amphfiers has been 

 the problem of coupling the plate of one tube, at a potential of a 

 himdred volts or so, to the grid of the tube in the next stage, which is 

 usually run at ground potential. Many ingenious devices have been 

 worked out for doing this, most of which are unsatisfactory for one 



