Physics Lab Equipment

LEMI-23 Pohl's Pendulum - Forced Oscillation

LEMI-23 Pohl's Pendulum - Forced Oscillation


  • Good system stability and smallamplitude attenuation at free oscillation

  • Obvious resonant effect and clear physical phenomena

  • Rich experiment contents, easy operation and high measurement accuracy


The phenomenon of forced vibration resonance has caught great attention in machinery, construction, and other engineering and scientific fields. It could be either harmful or useful depending on the actual situations. Many electro-acoustic devices are designed and manufactured by using resonance principle. In micro-science, resonance is also an important research tool, such as the use of NMR (nuclear magnetic resonance) for the structural study of materials. The representations of a forced vibration are the natural phase-frequency characteristic and the amplitude-frequency characteristic.


This apparatus consists of a mechanical resonant vibration unit and an electric control unit. A copper circular balance wheel is installed on a rack. One end of the spring is connected to the shaft of the balance wheel and the other end is fixed on the rack post. Under the impact of the spring force, the balance wheel swings freely around its shaft. There is a pair of permanent magnets beneath the rack with the balance wheel located in the magnet gap. Due to electromagnetic induction, when the balance wheel cuts magnetic lines, it is subject to an electromagnetic damping force. The damping magnitude is changed by changing the location of the magnets. To change from a forced vibration to a balance wheel rotation, an eccentric wheel is mounted on the motor shaft through a connect-rod mechanism to drive the balance wheel. A plastic glass wheel with engraved marker line is mounted on the motor shaft and rotates with the motor. The phase difference φ can be read out on the disk. The rotation speed of the motor can be precisely adjusted by the control box.

When forced vibration occurs, the phase difference between the balance wheel and the external force is measured using a flash lamp. When the black marker line passing the photoelectric gate, a flash is caused. Using this apparatus, the following experiments can be conducted:


1. Determine the amplitude and the phase-frequency characteristics of forced vibration

2. Study the influence of damping coefficient on forced vibration and observe resonant vibration phenomenon

3. Determine the properties of a moving object using frequency-flash method


Spring stiffness coefficient KVariation of free vibration period:<1%
Time measurementAccuracy: 0.001 s; error of period: 0.2%; 4-digit display
System dampingAmplitude attenuation<2% without electromagnetic damping
Amplitude measurementError: ± 1゜
Motor rotational speedRange: 15 ~ 50 r/min; period adjustable: 0.2 ~ 4 s
Phase difference measurementError < 2゜when phase difference between 40 ~ 140゜

Parts List

Main unit1
Electric control unit1
Wire and cable3

LEMI-23 Pohl's Pendulum.png

Amplitude versus frequency

LEMI-23 Pohl's Pendulum.png

Phase versus frequency

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