Primary cell terminology

Primary cell terminology offers much scope for confusion. When a primary cell is in use there are two circuits:

The external circuit, e.g. a lamp connected by wires to the cell terminals
The internal circuit via the liquid or paste in the cell

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The terminology relates to the internal circuit and some terms will be the opposite of those which relate to the external circuit. To use a computer analogy, suppose that computers A and B are connected by a serial data lead. Data out from computer A will be data in to computer B and vice-versa. Thus, if a wire is labelled "data out" or "data in" it is important to know whether this is in relation to computer A or computer B.

According to Ayrton and Mather:

In primary batteries the plate which is dissolved when the current flows is called the "positive plate", for the current passes through the liquid from this plate to the one unattacked, the latter being called the "negative plate". As, however, the current in the outer circuit passes from the unattacked plate to the one dissolved, the terminal on the former is called the "positve terminal" and that on the latter the "negative terminal".

To summarize, the positive terminal is attached to the negative plate and the negative terminal is attached to the positive plate. A further source of confusion is that Ayrton and Mather, writing in 1911, were assuming that current flows from positive to negative but nowadays it is considered that the electron flow is from negative to positive.

Nowadays the positive plate (negative terminal), which is usually zinc, is called the anode. The negative plate (positive terminal), which is usually carbon, is called the kathode or cathode. This may help to reduce confusion.