Voltage can vary across different points in a circuit, but determining the current at any point can be found without an ammeter, by using Ohm’s law.
If resistance stays the same but voltage rises, then the greater force pushes more current through the circuit. If resistance stays the same but voltage decreases, then less current will flow through the circuit. If voltage and amperage are recorded each time they change, and each voltage is put over each amperage, the resulting fraction always equals the same number.
That means that the total resistance of a circuit in ohms always equals the voltage divided by the amperage. This is Ohm's law.
R stands for resistance, V for voltage, and I for current in amperes. Therefore:
With Ohm's law, as long as any two of the three quantities are known, the third can be calculated.
For instance, instead of breaking into a circuit to measure current with a meter, if the voltage and resistance are known, Ohm’s law may be used.
Battery voltage can be measured – say, 12 volts. The value of the resistor is on its casing, say, 4 ohms. Current then equals voltage, 12 volts, divided by resistance, 4 ohms. Therefore 3 amperes of current flows through every point in the circuit.