The people – the physicists – we have encountered here all used experimental procedures and activities to arrive at their conclusions (often wrong, of course). They took notes of their findings, pondered the reasons why they were observing actions and reactions, and then published and discussed their observations and conjectures. We’ve already seen how clever some were. For instance, the torsional balance used by Coulomb was extraordinary. To have been able to measure, not absolutely but relatively, the tiny differences he noted in the charges placed on materials that he then carefully moved about to determine the inverse square of the distance relationship of charge repulsion was extraordinarily beautiful work.

As use of Volta’s battery or versions of the pile became more popular, it was possible to replicate and then extend the work of others because their results and findings were repeatable so that knowledge could be verified and then extended. The investigators noticed that their circuits and apparatus, interconnected by various materials and mechanisms, often developed heat within the assemblies. The German physicist Georg Ohm, in 1827 published his observational findings concerning voltage, current, and lengths of wire. His findings reported that readings from a galvanometer (a device which measures current and invented by Hans Oersted in 1820) were directly proportional to the length, diameter, and type of wire and the voltage applied to the circuit. Today we know that relationship as I = E / R, R = V / I, and E = I x R [1]. This equation, known as Ohm’s law, was agreed upon in 1861. It is extremely important and needs to be committed to memory.

When I was a kid, I asked a fellow how I should go about learning electronics. He said simply, “all you need to know about electronics is Ohm’s Law”. Interestingly, he was right. It was not at all obvious, then, how that could be. Ohm’s Law is so simple – just three terms. I was bored with it in five minutes. Little did I know then!

The variation in voltage and current due to the length, type, and diameter of wire came to be called the *resistance* to the flow of electronic charge. The definition was set as a proportional relationship between voltage and current and measured in units to be known as ohms. One ohm is defined to be the resistance in a circuit passing one amp of current with a potential difference of one volt. The majority of electronics is devoted to this simple and inseparable relationship. The manipulation, measurement, and response to these three values: current, voltage, and resistance.

Resistance became quite useful for regulating the amount of voltage and current in a circuit. Soon, devices termed resistors, made from various lengths and diameters of wire became available to experimenters. Lengths of wire made from different metals offered a wide variety of resistance values in many different physical forms. It was discovered (and patented in 1905), that wire made from a combination of iron, nickel, and chromium (to be called nichrome), offered both very high resistance and very high heat tolerance. Nichrome wire became very common for use in all types of heaters, most commonly, toasters. Carbon powder, compressed into small cylinders, sorted into like values and labeled by their value, became widely commercially available as devices known as resistors. There are now a huge variety of resistors made from sputtered metal films, carbon films, wire, ceramic, and conductive plastics. All have various characteristics which are useful in all kinds of circumstances. Some resistors vary their value of resistance based on other physical characteristics such as heat and light. Heat dependent resistors are useful for varying the amount of current and voltage in a circuit (which can be measure with a meter) based upon the temperature that the resistor is being subjected to – a thermistor. Photocells are light dependent resistors useful for measuring either the amount of light present, or just the presence or absence of light. When used to detect the presence of light, a photocell can be used to trigger door openers, turn on lights when it becomes dark, or count items interrupting a beam to make sure the right number of candy bars fill a box. Various forms of resistors are all around us performing useful functions.

The relationship between voltage (E), current (I), and resistance (R) can be represented by the simple figure below.

E, the symbol for voltage is placed over the I, symbol for current and R, symbol for resistance. You can remember then that to find the voltage value E, you can cover it with your finger which will then leave the I and R side by side, meaning to multiply them together – you need to multiply current by resistance. In a similar manner, to find out the current, cover the I with your finger leaving the E to be divided by R. To find the resistance when given the voltage and current, cover the R and see that you need to divide the voltage E, by the current I. Hopefully, you can remember this representation of the three values and their orientation, then be able to recall how to find the needed value given the other the other two.

[1] E = voltage, I = current, R = resistance

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