Physics Lab Equipment

LEMI-23 Pohl's Pendulum - Forced Oscillation

LEMI-23 Pohl's Pendulum - Forced Oscillation


  • Good system stability and small amplitude 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. Angle scales are marked on the outer ring of the balance wheel. 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 in the lower part of the rack and the balance wheel is 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. In order to make the balance wheel to do forced vibration, an eccentric wheel is mounted on the motor shaft through a connecting-rod mechanism to drive the balance wheel. A plastic glass wheel with engraved marker lines 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 unit.

When forced vibration occurs, the phase difference between the balance wheel and the external torque is measured using an LED flashlight. The flashlight is controlled by the reflective photoelectric gate of the balance wheel. Whenever the black line on the inner side of the balance wheel passes through the reflective photoelectric gate, a flash is triggered. Under stable conditions, the plexiglass pointer can be seen under the illumination of the flash as if it has been "stopped" at a certain scale. This phenomenon is called stroboscopic phenomenon, so this value can be easily read directly, and the error is not greater than 2°. 

The LEMI-23 Pohl’s resonance apparatus is used to quantitatively measure the amplitude-frequency characteristics and phase-frequency characteristics of mechanically forced vibration. The stroboscopic method is used to determine the dynamic physical quantity -- phase difference. The content of data processing and error analysis is also relatively rich. Using this apparatus, the following experiments can be conducted:

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

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

3. Learn to use the stroboscopic method to measure certain quantities of moving objects

The instruction manual contains experimental configurations, principles, step-by-step instructions, and examples of experiment results. Please click Experiment Theory and Contents to find more information about this apparatus.


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

Copyright © Lambda Scientific Systems, Inc. 2010-2024. All rights reserved.