NATURE OF QUANTITY MEASURED 



increased in order that the flux density be reduced. The effect is less serious 

 with penwriters because these have a substantial air gap. 



NATURE OF QUANTITY MEASURED 



Finally, a point important in electrophysiology, the single-sided amplifier 

 measures potentials or potential changes with respect to earth. It cannot 

 measure the difference between two potentials neither of which is earth. 



All the difficulties enumerated in this chapter may be overcome or mitigated 

 by the 'double-sided' ampHfier, and this will be studied next; before we can 

 do so, however, we shall have to take a superficial look at the vast subject 

 of negative feedback. 



163 



