Electrical Instruments

 

Electrophysiological measurements should satisfy two requirements:

(1)They should accurately measure the parameter of interest, and

(2) they should produce no perturbation of the parameter.

 

The first requirement can be discussed in terms of a voltage divider. The best way to measure an electrical potential difference is to use a voltmeter with infinite resistance. To illustrate this point, consider the arrangement of:

 

 

 

 

This Figure can be reduced to the equivalent circuit of:

 

 

Instruments used to measure potentials must have a very high input resistance Rin.

Before making the measurement, the cell has a resting potential of E,

This is measured with an intracellular electrode of resistance Re.

 

To understand the effect of the measuring circuit on the measured parameter, we will pretend that our instrument is a "perfect" voltmeter (i.e., with an infinite resistance) in parallel with a finite resistance R in , which represents the resistance of a real voltmeter or the measuring circuit.

 

The combination Re and R in forms a voltage divider, so that only a fraction of E appears at the input of the voltmeter; this fraction equals E Rin /(R in + R e ).

 

--> The larger Rin, the closer V is to E.

 

Clearly the problem gets more serious as the electrode resistance Re increases, but the best solution is to make Rin as large as possible.

 

On the other hand, the best way to measure current is to open the path and insert an ammeter.

If the ammeter has zero resistance, it will not perturb the circuit since there is no IR-drop across it.