Which of the following statements are correct about the errors in PMMC instruments?

I. The swamping resistance is included in series with the moving coil to reduce errors.

II. The swamping resistance is included in parallel with the moving coil to reduce errors.

III. Errors are caused by weakening of the permanent magnet due to aging.

IV. Errors are caused by weakening of the spring due to aging and temperature.

Errors in PMMC Instruments:

Permanent Magnet Moving Coil (PMMC) instruments are sensitive to various types of errors. Here are some points regarding the errors and the measures taken to reduce them:

1. The swamping resistance is included in series with the moving coil to reduce errors caused by temperature variations. This resistance is usually made of a material with a low temperature coefficient, such as manganin, to ensure that the total resistance remains relatively constant with temperature changes.
2. The swamping resistance is not included in parallel with the moving coil to reduce errors. This is incorrect because the purpose of swamping resistance is to stabilize the total resistance in series with the coil.
3. Errors are indeed caused by the weakening of the permanent magnet due to aging. Over time, the magnet may lose some of its magnetism, leading to inaccuracies in the instrument readings.
4. Errors are also caused by the weakening of the spring due to aging and temperature. The spring provides the necessary restoring force for the coil, and any change in its properties can affect the accuracy of the instrument.
   
    

The correct statements about the errors in PMMC instruments are: I, III, and IV