As we pointed out before, all parts of a turntable interact with each other. Just as the cartridge depends on the arm, the arm depends on the chassis. There are a variety of esoteric arms but most can be broken down into three categories:
Linear Tracking - the arm tracks a straight line keeping the cartridge perpendicular to the radius of the record at all times. There are benefits but largely speaking linear tracking arms are complicated and expensive to build and fit in a way that takes advantage of their benefits. In the interest of conserving time and space, we're going to leave linear tracking behind us.
Bearing Pivot - The standard tonearm design. The arm pivots on two bearings of various designs.
Unipivot - A single pivot point usually a small spike resting in a small cup. In a unipivot design, the weight of the arm is supported purely by the stylus on the record and the pivot point resulting in a coupling area that's almost infinitely small. A unipivot relies on very careful design and manufacture to maintain proper balance and tracking. Although a small percentage of arms made are unipivots we mention them because the Roksan Radius turntable uses a unipivot and our favorite arm ever, the Naim Aro is a unipivot.
Although many of the features discussed below will apply to all arms, we're sticking strictly with conventional bearing designs.
A tonearm comprises the following:
Headshell - The attachment point for the cartridge, comprising holes for mounting screws and hookup wires. Most of the time the headshell is detachable but better arms often use a fixed headshell in the interest of rigidity and minimal connections in the wiring.
Arm Wand - the tube between the pivot point and the headshell
Bearing - The arm is a teeter-totter. The bearing is where it pivots both up and down and side to side.
Counterweight - the small mass of the cartridge on the end of the long arm wand is balanced on the other side of the pivot by a heavy mass of the counterweight on a short "arm stub".
Tracking Force Adjustment - often simply a matter of moving the Counterweight fore and aft but sometimes a separate dial using a spring to apply downward force between the cartridge and the record.
Anti-skate - Also known as Bias. The physics of a curved groove moving past the stylus puts greater pressure on the inner wall of the groove. With the low tracking forces used by modern cartridges, this creates channel imbalances and tracking problems. The anti-skate force, usually a weight on string, spring or magnet applies a very gentle pressure towards the outside to center the needle in the groove.
Suffice to say human ingenuity has created many ways to move a cartridge across a record but most designers have settled on a rigid, medium mass arm with a straight arm want and a captive headshell. The finest value in tonearms today is Rega's line comprising the RB250, 300, 700, and new top-of-the-line RB1000. Rega's arms use a single piece casting of aluminum comprising the headshell, fingerlift, arm wand, and bearing housing. Like many modern designs Rega figured out a wide diameter, thin walled arm tube provides the greatest rigidity to weight ratio. The complex metallurgy involved has won them awards outside the hi-fi industry. The essence of a very rigid arm tube is to allow the cartridge to move freely on two axis yet hold it torsionally rigid which maximizes the speed at which information on the record is transformed into a musical signal. In other words, less of the vibration is transferred into twisting or oscillation of the cartridge body/arm system so the musical signal created by the cartridge is closer to the information on the record.