PhyE&M...voltmeter

Q: 我想知voltmeter係點樣運作?我只知voltmeter係量p.d.,但係佢點樣量度?

The operation of a voltmeter is based on the principle of Ohm's Law.

The core of a voltmeter is a moving coil galvanometer. In order to measure voltage, a high resistance resistor is connected in series with the moving coil metre (this resistor is called a multipler). The "resistor and moving coil metre arrangement" then forms a voltmetre. By Ohm's Law, assuming the internal resistance of the metre is negligibly small compared with the connected resistor,

V = I.R, where V is the voltage across the connected series resistor R, I is the current flows through the metre.

Hence, the voltage across R (which is, to a good approximattion, the voltage to be measured), is proportional to the current I that passes through the metre.

Q: 同埋佢電阻值由無限大到低電阻會點影響佢個reading?

As explained above, for a fixed current passing through the moving coil metre, the value of V (the voltage to be measued), depends very much on the resistance of the resistor connected to the metre. Using a high resistance enables high voltage to be measured. Connecting a low resistance enables a low voltage to be measured with higher precision.

Having said that, the resistance used in the voltmeter shall always be high as compared with the resistance in the circuit to be measured in order to not interfere with the original condition of the circuit.

voltmeter係量度電壓儀器，分指針式及數字式不同運作，要量度電壓，只要將電壓錶並連測試點，例如電池端點，電路端點。直流電壓錶有分正負極，通常正極係高電位一端，而負極就係低電位一端。以指針式為例，量度電壓時，電壓錶要取用部份電路電流，因為電壓錶內有線圈，線圈通電產生磁性令指針擺動，為了不影響電路阻抗，一般電壓錶都有較高內阻，以普通萬用錶為例，直流內阻係以20kohm/v，用10v量程計內阻就係200kohm，如果電路阻抗低於200kohm則影響不大，因為大部份電流都流經電路，但如果電路阻抗高達1mohm，則影響相當大，因為量度電壓係用並連方式，1mohm並連200kohm，等效電阻係細於200kohm，這樣大部份電流就會流經電壓錶。試試計一計，如果1mohm電路有10vpd，電流等如0.01ma，並連電壓錶後，等效電阻降至167kohm，算一算pd只有1.67v，誤差高達83%。