Circuit on the breadboard
Like real parts, each component's true value differs a little from its label, so your measured τ will not match the nominal RC exactly.
Function generator
Measuring τ. With a square wave slow enough that the capacitor fully charges (half period ≥ 5τ), set time cursors at the rising edge and where VC reaches 63% of its step; or set voltage cursors at 0% and 63% and read the time between them. Now raise the frequency: when the half period gets shorter than τ the capacitor never finishes charging, and VC turns into a small triangle around the mean. The circuit is a low-pass filter, the same reason the membrane smooths fast synaptic inputs. In the model cell, Rm leaks current, so VC settles at Rm/(Rs+Rm) of the input and τ = C·(Rs ∥ Rm) is shorter than RsC. The scope input is treated as ideal (it draws no current).
Recording stage
Test pulse
Model: the preparation
The true properties of the pipette and cell. In a real experiment these are unknown; the membrane test estimates them.
Pipette & seal
Cell
Amplifier: compensation
Settings you dial in on the amplifier to cancel capacitive transients. When they match the preparation, the transient disappears and the dials read out its values.
Cancels the pipette capacitance Cp. Too much gives an inverted spike.
Amplifier: recording
Membrane test results
| Parameter | Measured | True |
|---|