Current is the workhorse of electricity, what in fact we have in mind when we imagine electricity. Just as in the faucet in your house. Open the faucet and water flows, it has currrent. Current is what charges the battery, what flows through the headlight's filiment, what creates the magnetism inside the horn or the starter solenoid. It's what makes things work. Anyone who struggles with grasping electricity needs to become a friend of current. Current is probably the most faithful proof of the unseen world of electricity because unlike other measurements which merely *predict* electrical action, current is itself a snapshot of that action. Your multimeter reads current as amps, a word that comes from Andre Ampere, the late eighteenth century French mathematician credited with forming the theories of electromagnetism that resulted in his creating the world's first ammeter, or measurer of electrical current.
Voltage is a meter measurement of electromotive force (EMF). Just as the physical world has force (weight, pressure, gravity), the electrical world does also, it has EMF. Voltage is not power and it is not movement. It is not even electricity in the strictest sense, so it has got in my view, rather ironicallly, way too much "press.". Take a wooden board. Prop the board up on a brick, and ask yourself, will a marble roll down the board? Of course. You don't even have to test it, you just know it will. The angle of the board assures you of that. Voltage is exactly like this. It is simply what results when two points, one higher than the other, are created. A latent (that is, potential, untested) force results. Volts is static in other words, like water pressure. Nothing has to be happening for voltage to exist. The eighteenth century Italian physicist Allesandro Volta, the inventor of the battery, gave us the term, volt.
Resistance is the electrical equivalent of friction. It's a good thing sometimes and a bad thing at other times, just like the friction of physics. Physical friction is good when it is used in the brake pad to brake disc junction to slow a vehicle, but bad when the throttle cable gets stiff because of rust and dirt. Electrical resistance ks a good thing when it is used in a headlight filament to get the glow of the headlight's bright beam. It's a bad thing when the connector to the headlight is corroded and the headight stops working.
Resistance measurements are standard troubleshooting technique in most electrical disciplines. They're easy to do, and in the perfect world the engineer's resistance specs make sense. However, in the imperfect world, using design specs for troubleshooting just doesn't work, and resistance tests are the least reliable of all available test techniques. In the rough and dirty world of powersports, where the variables are much greater than is typical in scientific endeavors, resistance measurements are heavily discounted and powersports service experts use them hardly at all, preferring instead dynamic, working electrical tests, primarily current tests. Meters measure resistance in ohms, a word borrowed from the eighteenth century German physicist Gorge Simon Ohm, who first proposed a relationship between current and EMF.
The relationship ol' Georg developed is called Ohm's Law. Naturally, with these three dimensions of electricity being so important, you might expect there to be an established principle governing them. Current, voltage and resistance are in fact intertwined, a fact that Ohm's Law expresses by saying two things. First, that voltage and current work together, they have a "direct" relationship. Back to our board analogy. Prop it up so it is higher than it was before. Will the marble roll faster than before? You don't even have to test it to see, do you? Of course it will. Everything else bring equal, more pressure (voltage) results in more movement (current). How about when the board is made lower than before? Won't that make the marble roll slower? Ohm's Law also declares that less electrical resistance permits more current flow and more resistance reduces that flow, which is known as an "inverse" relationship. So the three -- EMF, current and resistance -- affect each other.
To sum up, current measured in amps is active, moving electricity. It gets its push from EMF which is merely electrical strain measured in volts. Its movement is opposed by electrical friction called resistance which is measured in ohms. Ohm's Law says one volt will push one amp through one ohm. It also proves that reducing that ohm to a half while keeping the voltage constant will double the current. Make sense? Pretty basic arithmetic.
Watts is a fourth piece, the odd man out. Not a measurement available on your multimeter, but a calculation, watts is power. Not voltage, not amps, but the two combined. So much pressure combined with so much movement, equals watts. The interesting thing about watts is this is how we compare charging systems. Just as we compare audio systems, we compare the power of vehicle charging systems in watts. Powersports vehicles once averaged some 150 watts of alternator power. Today that number is closer to 600, with many larger touring bikes pumping out twice that. James Watt, the mid-eighteenth century Scottish mechanical engineer who also gave us the term "horsepower", lends his name to the watt.
Speaking of watts and horsepower, comparing the two reveals the links between the physical and electrical worlds. In most countries outside of the U.S. motor vehicle engines are rated in kilowatts, not horsepower. They are virtually the same thing. Let's break this down. A shovel-full of dirt is force. If the shovel-full is moved, force becomes work. Once the shovel-full's work is timed in seconds, guess what? We have power. The same is true in electricity. EMF or volts, the electrical "weight" resulting from the difference between two points, is the force. Moving this force happens with current, which is a measurement of movement and time together. A little different combination, but the same ingredients algebraically, and the same result. To simplify it, horsepower is force x distance (i.e. work, which we know as torque) divided by time. Similarly, watts measures volts (i.e. force) x current (distance divided by time), algebraically reorganized, but the same elements. In both watts and horsepower, the basic equation is the same: force x distance divided by time.