Balances are different from scales in that they have a single bar resting on a pivot with two identically weighted platforms on either end of the bar. When you place a weight of known value on one platform, and another object of unknown weight on the other. To find the exact weight of the unknown, you simply adjust the amount of weight on the known side of the balance. This makes it sound like there is only one type of balance, but there are more than just one. There are actually three types of balances: the equal arm balance, the unequal arm balance, and the spring balance.

Equal arm balance

The equal arm balance is named such because the distance between the pivot and each end of the bar are equidistant, or separated by an equal length from the pivot point. This pivot point, called the fulcrum, sits on the exact center of the bar. This means the fulcrum is at the center of gravity, and the balance will be “zero” when the platforms are at equal levels. This equality is achieved by having to two things of the same weight on each platform.

Unequal arm balance

As you would expect from a name like that, the fulcrum is not equidistant from the both platforms. Instead, one arm is shorter than the other. The object to be weighed is placed on the shorter end, while the known weights are moved along the longer arm until balance is obtained. A steelyard is a example in common use today. It's used in underdeveloped nations because it's inexpensive, easy to port, and the length of arms can multiply the effects of a smaller weight by a factor of 100, it is easy to use a small took for a large item.

Spring balance

The spring balance consists of a coiled spring at one end, with a hook at the other end for the object that is to be weighed. This uses physics of a springs elasticity to calculate the weight of the object. You see, the distance that the spring is stretched is proportional to the weight of the object that is being weighed. A pointer and a graduated scale are attached to the balance to convert the distance the spring is stretched into a weight reading.

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