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The sensitivity of the D’Arsonval-type meter, and of similar designs, depends on several factors. First is the strength of the permanent magnet (if the meter uses a permanent magnet). Second is the number of turns in the coil. The stronger the magnet, and the larger the number of turns in the coil, the less current is needed in order to produce a given magnetic force. If the meter is of the electromagnet type, the combined number of coil turns affects the sensitivity. Remember that the strength of a magnetomotive force is given in terms of ampere-turns. For a given current (number of amperes), the force increases in direct proportion to the number of coil turns. The more force in a meter, the greater the needle deflection for a given amount of current, and the smaller the current necessary to cause a certain amount of needle movement. The most sensitive ammeters can detect currents of just a microampere or two. The amount of current for full-scale deflection (the needle goes all the way up without banging against the stop pin) can be as little as about 50 μA in commonly available meters. Sometimes, it is desirable to have an ammeter that will allow for a wide range of current measurements. The full-scale deflection of a meter assembly cannot easily be changed, because that would mean changing the number of coil turns and/or the strength of the magnet. But all ammeters have a certain amount of internal resistance. If a resistor, having the same internal resistance as the meter, is connected in parallel with the meter, the resistor will draw half the current. Then it will take twice the current through the assembly to deflect the meter to full scale, as compared with the meter alone. By choosing a resistor of just the right value, the full-scale deflection of an ammeter can be increased by a large factor, such as 10, or 100, or This resistor must be capable of carrying the current without burning up. It might have to draw practically all of the current flowing through the assembly, leaving the meter to carry only 1/10, or 1/100, or 1/1000 of the current.

http://slideplayer.com/slide/4158558/

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