Definition: The box which contains the resistors of different values for estimating and comparing the resistance is known as the 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 main advantage of the resistance box is that the variable resistances are available at one point. If any circuit requires variable resistances, then there is no need of replacing the resistor. The circuit is directly connected to the resistance box, and by changing the rotary switches, the variable resistances are obtained.
The resistance box is of three types. They are
- High resistance Box
- Low Resistance box
- Fractional Resistance Box
The value of the high resistance box lies from 1Ω to 5000Ω or above while the value of the low resistance box is between 1 to 500Ω. In fractional resistance box, the value of resistance is in the form of a fraction. The range of fractional box lies between 0.1Ω to 50Ω.
The construction of the box is simple and cheap. The resistance box is available in different designs. It is also used for testing and designing the circuit in the laboratory.
Simple Resistance Box
The simple resistance box has two copper terminals for connecting the positive and negative terminal of the 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.
On the second side of the ebonite sheet, the resistances of different value are connected in series with each other. For connecting the resistance across the circuit, the knobs of the particular resistance need to be removed. When all the knob is placed on the air gap then the current pass through the copper stud, no resistance is connected to the circuit.
The process of using the resistance box.
- The value of resistance is kept very high so that the less power dissipation occurs in the connecting circuit.
- Before connecting the box to the circuit, it is essential to set the value of the circuit to minimum resistance. So the less dissipation takes place in the circuit. The resistance of the box is either equal or greater than the resistance of the circuit.
- The resistance box is always connected to the circuit by the help of the plug connectors.
Decade Resistance Box
In resistance box, the resistor is fixed inside the box. They are arranged in such a manner that the value of resistance varies at every step. The box consists the rotary selector switch. The variable resistances are obtained by rotating the selector switches. The key plug is also used for selecting the resistances but rotary switches are more suitable. Hence it is mostly used in the resistance boxes.
Consider the examples of normal decade resistance box. The normal arrangement of the rotary switches in a decade resistance box is shown below.
- A range of Switch one – 1 to 10 ohms
- A range of switch two – 10 – 100 ohms
- A range of switch three– 100 – 1000ohms.
- A range of switch four – 100 ohms and above.
The resistance box contains more than one rotary switch shown above in the figure. The resistance of each of the selector switches varies from few ohms.