For instance, in the electrical circuit shown in fig. 39, the potential at the point _a_ is higher than that at _m_, that at _m_ higher than that at _n_, etc., just as in the water circuit, shown in fig. 38, the hydrostatic pressure at _a_ is greater than that at _m′_, that at _m′_ greater than that at _n′_, etc. The fall in the water pressure between _m′_ and _n′_ (fig. 38) is measured by the water head _n’s_. 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: 1. Open circuit cells; Used for _intermittent work_, where the cell is in service for short periods of time, such as in electric bells, signaling work, and electric gas lighting. If kept in continuous service for any length of time the cell soon polarizes or “runs down,” but will recuperate after remaining on open circuit for some little time. 2. Closed circuit cells. This type of cell is adapted to furnishing current continuously, as in telegraphy, etc. With respect to the second method, cells are classified as: 1. One fluid; 2. Two fluid; =Ques. Describe a primary cell.= Ans. A primary cell consists of a vessel containing a liquid in which two dissimilar metal plates are immersed. In _one fluid_ cells both metal plates are immersed in the same solution. In _two fluid_ cells each metal plate is immersed in a separate solution, one of which is contained in a porous cup which is immersed in the other liquid. =Ques. What name is given to the metal plates?= Ans. They are called _elements_. =Ques. What is the fluid called?= Ans. The _electrolyte_ or _exciting fluid_. The term “electropoion” is a trade name for the electrolyte employed in the Fuller cell.