The experiments today deal with circuits that have some unknown component parameter, such as an unknown voltage supply or a resistance that one could not directly measure. Resistances that can’t be directly measured actually come up a lot in experimental physics, though today we will keep it simple with a powered light bulb. The resistance of the tungsten wire in the bulb will change as the current heats it, so measuring the resistance of the unplugged bulb isn’t meaningful here.

In order to measure these unknown parameters, we are going to actively change one of the components of the circuit (a potentiometer) and use a very sensitive current-measuring device (a galvanometer) to balance two halves of a circuit.

2.1 Potentiometer

A potentiometer is basically a variable resistor. The potentiometer (Figure 1) has three terminals: one at either end of the full resistor, and another connected to a ‘wiper’ that can be dragged along the length of the resistor. If one measured the resistance between one end of the potentiometer and the wiper, then the resistance would change as the wiper was swept over its range. Now we don’t have to swap resistor components to change our circuit!

Figure 1: Schematic of a potentiometer. As the wiper rotates, the resistance between the wiper and either end will change. The total resistance between lugs 1 and 3 remains constant. Figure from electronics-tutorial.ws.

2.2 Galvanometer

A galvanometer is a device that directly measures incredibly small amounts of current; the ones in the lab have a ±20 or ±50 A scale (See Figure 2). This small amount of current travels through a coiled wire in the field of a permanent magnet. You will learn all about the forces that result from wires carrying current in magnetic fields in lecture. For now, please believe me that the coiled wire rotates in the magnetic field. The coiled wire is attached to an indicator-needle and a calibrated scale is added behind the needle to make a neat little gauge. Galvanometers are useful teaching tools since they use fundamental E&M principles for their operation, but you won’t see them in the research lab, or almost anywhere else (Practical Electronics for Inventors does not even mention them). So enjoy your time with your galvanometer while you can!

Figure 2: (a) A schematic drawing of a galvanometer. The current travels through the wound wire in the magnetic field, causing the indicator-needle to rotate. (b) One of the galvanometers we have in the lab.

2.3 Ohmmeter

You will be using the ohmmeter function on your handheld multimeters to measure some resistances. The ohmmeter has a precise current source built in. This known current passes through the unknown resistance, generating a voltage because OHM’S LAW. The voltage charges an internal capacitor, and the time it takes to discharge is used to determine the voltage. The really important thing to note here is that the ohmmeter is really measuring the voltage across the resistor that results from the internal current source.

2.4 Kirchhoff’s Law

Kirchhoff’s law has two forms: the current form and the voltage form. The current law, called the 1 law, says that the sum of all currents going into a node in a circuit diagram (as in Figure 3) must be equal to the sum of all currents leaving the node:

(1)  

.
In Figure 3, Kirchhoff’s Law tells us that . The voltage form of Kirchhoff’s Law, called the

Figure 3: Kirchhoff’s current law. The sum of currents into a circuit junction is equal to the currents out of the junction. Figure from Practical Electronics for Inventors.

2 law, states the potential difference around a complete loop in a circuit (as in Figure 4) must equal zero:

(2)  

Figure 4: Kirchhoff’s voltage law. The sum of voltages around the loop will be zero. Figure from Practical Electronics for Inventors.

These laws are helpful in determining unknown currents or voltages in complex circuits. There is no end to the number of Kirchhoff-inspired homework problems out there. Practical Electronics for Inventors has some good ones, as does E&M by Griffiths.