# Current electromotive force and resistance

## To measure the internal resistance of a cell

To measure the internal resistance of a cell A high-resistance voltmeter is connected to the cell terminals and the reading noted. This will give the e.m.f. of the cell. A 1 n resistor is now connected to the cell terminals and the voltmeter reading taken again (Fig. 35.10). Further readings are taken using 2, 3, 4, etc., …

## “lost volts” when a cell is producing current in a circuit

“lost volts” when a cell is producing current in a circuit For practical purposes, the e.m.f. of a cell may be measured to a very close approximation by taking the reading of a high-resistance voltmeter connected directly across the cell terminals when the cell is not connected to anything else. The reason why this value is only approximate …

## Arrangement of cells

Arrangement of cells A group of cells connected together is called a battery. Normally, cells are connected in series, the positive of one being connected to the negative of the next and so on (Fig. 35.7). On occasions, however, they may be connected in parallel, i.e., all the positives connected together and the same with the negatives …

## Special case of two resistors in parallel

Special case of two resistors in parallel The formula obtained above may be extended for any number of resistors. In practice, however, it is rare to find more than two resistors in parallel, and therefore we shall examine this case in more detail. For two resistors R\ and R2 in parallel

## Practical importance of the internal resistance of a cell

Practical importance of the internal resistance of a cell Cells of one particular kind, for example, dry cells, may be obtained in several different sizes, but so long as they are all made of exactly the same materials and have the same strength of electrolyte they will have identical e.m.f.s. The strength of the current which is …

## Resistors in parallel

Resistors in parallel Resistors are said to be in parallel when they are placed side by side and their corresponding ends joined together (Fig. 35.6). The same potential difference will thus be applied to each, but they will share the main current in the circuit. We will suppose that the main current I divides into II, 12, and …

## Resistors in series

Resistors in series A number of resistors R), R2′ R3′ in ohms, are said to be connected in series if they are connected end to end consecutively so that the same current I, in amperes, flows through each (Fig. 35.5). If R is the combined resistance and V, in volts, is the total potential difference across the resistors, …

## Electric circuit. Use of ammeters and voltmeters

Electric circuit. Use of ammeters and voltmeters Fig. 35.3 illustrates some of the symbols used in electric circuit diagrams. When the terminals of a cell are joined to the ends of a resistor an electric circuit is formed. If several resistors are connected end to end in the circuit so as to form a continuous path they …

## Resistors

Resistors Radio and television receivers contain large numbers of resistors with resistances of anything from a few ohms to millions of ohms. Some are wire wound, while others are made from carbon or graphite. They are supplied with either wire ends or terminals for connection purposes. The special standard resistors designed for laboratory purposes are described in chapter 40.

## Effect of temperature on resistance. Semiconductors

Effect of temperature on resistance. Semiconductors The resistance of a pure metal increases with temperature but the resistance of certain other conducting materials, e.g., carbon decreases with temperature. Various metal alloys, notably manganin and constantan which are used for making standard resistors, show very small resistance changes with temperature under normal laboratory conditions. Certain other substances notably germanium, silicon, and …