**On the basis of working Principle the computer can be classified into: **

1. Analog computer

2. Digital computer

3. Hybrid computer

**» Analog computer **

Analog computer operates on inputs of continuously varying electrical voltage. It measures the input rather than counting. The name that is derived from the Greek word 'analog' denotes that the computer functions by establishing similarities between two quantities that are usually expressed as voltages or currents. Analog computers are powerful tools to solve differential equations. They are mainly used in scientific design and production environments. Each one has to be constructed to do a specific job and will respond very quickly to changes in the measured inputs.

**» Digital Computer **

A digital computer operates essentially by counting. All quantities are expressed as discrete digits or numbers and computations are done with numerical digits. Mathematical expressions are represented as binary digits (0 and 1) and all operations are done using these binary digits at a very high rate. The digital computer basically knows addition. Remaining operations, like multiplication and division, are first converted -into addition and then calculated. Digital computer is much faster than analog computer and the computations are far more accurate. They come in various sizes starting from pocket size to the large systems occupying big rooms. Most of the computers available today are digital.

**» Hybrid Computer **

Hybrid computer combines features of both analog and digital computers. In this computer, some calculations are done in the analog portion of the computer and some are done in the digital portion of it. In a hospital, for example, analog devices may measure patient's heart function, temperature and other vital signs. These measurements may then be converted into numbers and supplied to a digital component in the system. This component is used to monitor the patient's vital signs and to send an immediate signal to the nurse's station if any abnormal readings are detected.