Some form of amplitude control was available fairly early in the history of ultrasonic plastics assembly, even before ultrasonic welders had any other form of process control. Amplitude can be controlled either electronically or acoustically. Amplitude is typically defined as the peak-to-peak distance traveled by the work face of a transducer or tool. Sometimes amplitude is defined from rest to peak, and in fact amplitude can be measured at various places on a vibrating body in various directions, but it is essentially only the linear amplitude which is of any use in plastics welding, so we tend to limit our discussion to that. In most ultrasonic system design, voltage determines amplitude, so amplitude can generally be adjusted electrically by changing the output voltage to the transducer or converter. In many designs that predate the late 1980s (some of which are still on the market today), output voltage from the generator or power supply would be proportional to input voltage, which could cause process variations if line voltage fluctuations or variations occurred. The transducer or converter can be thought of as a reciprocating electric motor. The generator or power supply puts out a certain voltage, and then as the transducer is mechanically loaded (as resistance to motion is felt by the device), it will draw more amperage from the generator or power supply in an attempt to support the amplitude. Unless the generator is equipped to provide a compensatory increase in voltage, amplitude will actually sag under load, much as the speed of the electric motor on a power saw sags as the saw encounters resistance when cutting wood. Amplitude could also be controlled through manipulating the shape of the tooling components, which led to the creation of the booster.