# Ammeter Diagram

## Ammeter Diagram:

DC Ammeter : The PMMC galvanometer constitutes the basic movement of a dc ammeter. Since the coil winding of a basic movement is small and light, it can carry only very small currents. When large currents are to be measured, it is necessary to bypass a major part of the current through a resistance called a shunt, as shown in Fig. 3.1. The resistance of shunt can be calculated using conventional circuit analysis. Referring to Fig. 3.1 Rm = internal resistance of the movement. Ammeter Diagram …

Effect of Frequency Calibration : The Effect of Frequency Calibration arises because of various factors such as: Skin effect Non uniform distribution of current along the heater wires Spurious capacitive currents Skin Effect The skin effect causes a higher reading at higher frequencies, especially if the heater wire is small. A low current instrument with a circular cross-section, used in vacuum, may have a skin effect error of less than 1% at frequencies up to 30,000 MHz. Ribbon heaters are often used for large currents, but they have larger skin effects. Solid wire, and better still hollow conductors are ideal with a view to minimising from the …

Measurements of very Large Currents by Thermocouples : Measurements of very Large Currents by Thermocouples – Thermocouples instruments with heaters large enough to carry very large currents may have an excessive skin effect. Ordinary shunts cannot be used because the shunting ratio will be affected by the relative inductance and resistance, resulting in a frequency effect. One solution to this problem consists of minimising the skin effect by employing a heater, which is a tube of large diameter, but with very thin walls. Another consists of employing an array of shunts of identical resistance arranged symmetrically as shown in Fig. 3.7 (a). In. Fig. 3.7 …

Thermocouple Definition : Thermocouple Definition consists of a junction of two dissimilar wires, so chosen that a voltage is generated by heating the junction. The output of a thermocouple is delivered to a sensitive dc microammeter. (Calibration is made with dc or with a low frequency, such as 50 cycles, and applies for all frequencies for which the skin effect in the heater is not appreciable. Thermocouple instruments are the standard means for measuring current at radio frequencies.) The generation of dc voltage by heating the junction is called thermoelectric action and the device is called a thermocouple. Ammeter Diagram …