It consists of a single loop of thin phosphorbronze strip which forms the moving system. It is called a vibrator loop. The vibrator loop is situated in the field of a permanent magnet but when high sensitivities are required, highly saturated electromagnets are used.

The vibrator loop is formed by passing the strip over an ivory pulley. A spring tension adjusting device is attached to the ivory pulley. A small mirror for reflecting light is cemented to the loop midway between the bridge pieces. A Duddell element is essentially a vibration galvanometer having a low period of vibrations. The natural frequency is about 10kHz. The loop is immersed in oil to get high degree of damping.

The galvanometer units can be constructed with diameters 3mm to 6mm and about 75mm length. A typical unit is shown in fig. These galvanometer units are often called pencil galvanometers. They comprise the moving element of the galvanometer and are set in a fixed frame of the permanent magnets.

Two or more units per cm can be constructed and 50 or more galvanometers are included in a single oscillograph in order to record many waveforms simultaneously.

When an alternating current is passed through the loop, an alternating torque acts upon the loop and it vibrates. When a beam of light is cast upon the mirror, it is reflected. As the loop is vibrating, the reflected beam will move backwards and forwards. If a wave shape is to be recorded, a photographic film is passed at a constant speed in a direction perpendicular to that of the movement of the reflected beam of light. The actual speed of film depends upon the frequency of the wave being recorded.

The wave shape is recorded on the film as a result of the two perpendicular motions, of the film and the reflected light. The oscillograph is calibrated by passing a known value of direct current in the loop and recording the deflection produced by the current.

If the waveform is not to be recorded but is only to be observed, the arrangement as shown in fig is used. The reflected beam of light from the vibrator mirror falls on a plane mirror and is further reflected on to a screen. The plane mirror is rocked by a cam driven by a synchronous motor. The synchronous motor is supplied from the source whose waveform is to be observed. The plane mirror driven by the cam, gives forward motion of the reflected beam on the screen. The forward motion is carried out for 1½ cycles of the wave, and the plane mirror is returned to its original position during the next half cycle.

During this cycle the light is cut off by a rotating shutter. The process is repeated continuously and the wave shape appears stationary on the screen on account of persistence of vision.

Uses :

1. These oscillographs are particularly suited at low voltages and low frequencies.
2. These oscillographs may be provided with a larger number of elements and therefore a number of quantities may be observed or recorded simultaneously.
3. They afford convenient facilities for photographic purposes.

Disadvantages :

1. The use of these oscillographs is limited to frequencies upto 500 Hz if an accurate record of the waveform is required. Above this frequency the response to higher order harmonics is very small and therefore the harmonics are not recorded.
2. The use of these instruments is more complicated.

These oscillographs are not commonly used these days and for many purposes they have been replaced by the cathode ray oscillographs.