In order to measure the fall in electrical potential between m and n, (fig. 39), the terminals of a volt meter are placed in contact with these points as shown. Its reading will give the difference of potential between m and n, in volts, provided that its own current carrying capacity is so small that it does not appreciably lower the potential difference between the points m and n by being touched across them; that is, provided the current which flows through it is negligible in comparison with that which flows through the conductor which already joins the points m and n.
CHAPTER IV
PRIMARY CELLS
The word “battery” is a much abused word, being often used incorrectly for “cell,” as in fig. 40. Hence, careful distinction should be made between the two terms.
A battery consists of two or more cells joined together so as to form a single unit.
There are numerous forms of primary cell; they may be classified as follows:
1. According to the service for which they are designed;
2. According to the chemical features.
With respect to the first method cells are classified as: