I test the bio amp by recording my muscle activity from my forearm. Note the three electrodes: two differential ones and ground. All bio signals are recorded differentially with three electrodes where sometimes the negative differential input is derived from more than one electrode, for example, aVF,aVR/aVL or V1..V6 in ECG recordings. Sometimes such a setup is called “monopolar” but is still a differential recording.
The electrode cables in this clip were hand-made from “snaps” which click into studs which you can buy from any sewing shop. However later we used more or less exclusively these commercial electrode clips which are a bit larger but connect directly to a standard lab (“banana”) cable so that no soldering is required.
(!!) Note that at this stage the amplifier has no input protection which will be added in the final clip: