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# Long description

This shows two rectangular 9 volt batteries arranged so that their terminals are on the right. The batteries are placed one above the other so that the terminals from top to bottom read + (1), − (2), + (3), − (4). The − (2) and + (3) terminals (that is, the negative terminal of one battery and the positive terminal of the other) are connected to each other and the potential of the node is called Vmid. By convention, this is considered to be ground.

The top + (1) battery terminal is connected to the positive power supply terminal of an op-amp. This connection is called VS+ and is shown to have a positive voltage (9 volts) with respect to Vmid. The bottom − (4) battery terminal is connected to the negative power supply terminal of the same op-amp. This connection is called VS− and is shown to have a negative voltage (minus 9 volts) with respect to Vmid.