The box which contains the resistors which can cover a wide range of values for estimating and comparing the resistance is known as the resistance box.
Features of Resistance box
- The accuracy of the resistance box is very high.
- The main application of the resistance box is to control the specific value of current to flow through the circuit.
- The construction of the box is simple and cheap.
- The resistance box is available in different designs.
- It is used for testing and designing the circuit in the laboratory.
The resistors present in the resistance box are mounted in the box and are so arranged that the values of resistance can be varied in steps. The circuit is directly connected to the resistance box, and by changing the rotary switches, the variable resistances are obtained.
If any circuit requires variable resistances, then there is no need of replacing the resistor. This is the key advantage of using resistance boxes.
Types of resistance box
- High resistance Box → 1Ω to 5000Ω or above
- Low Resistance box → 1Ω to 500Ω
- Fractional Resistance Box → 0.1Ω to 50Ω
1. Simple resistance box
The simple resistance box has two copper terminals for connecting the positive and negative terminal of the circuit. It is shown in fig. 1 below. The red colour knobs to the extreme left side of resistance box are terminals for connecting circuit. The cover of the box on which terminals and knobs are placed is made by ebonite material. The knob is used for adding and removing the resistance from the circuit.
For connecting the resistance across the circuit, the knobs of the particular resistance need to be removed.
2. Decade resistance box
Generally, a rotary switch is much easier to use. There may be more than one selector switch in the resistance box.
The normal arrangement of the rotary switches in such a resistance box is shown below.
- A range of Switch one – 1 to 10 Ω
- A range of switch two – 10 – 100 Ω
- A range of switch three– 100 – 1000Ω
- A range of switch four – 100 Ω and above
Therefore, such resistance boxes are commonly called as decade resistance boxes. A decade resistance box is shown the fig. 2 below.