[A new actograph operating by capacitance sensing]

Capacitance sensing devices are useful for ecological and ethological studies (Zucker et al., 1968). If the location and activity of small animals (e.g. newts) is to be recorded, problems arise due to the very small changes in capacitance that must be detected reliably. By using a special bridge circuit (Fig. 2) an extremely sensitive detector can be constructed. With the output voltage and frequency of the RF-generator kept constant, the RF-current is determined only by the capacitance of the two sensors C₁ and C₂. This current is rectified by the transistors T₁ and T₂ and the difference is detected by a Schmitt trigger. Outside influences that affect both sensors in the same way will thus be eliminated, resulting in excellent long-time stability and sensitivity of the device.The sensors simply consist of copper foils, about 1 cm apart, glued onto an insulating, flat surface. A glass with the animal is placed on these sesors and no further connection is necessary. The change in capacitance is generated by the animals building a capacitance bridge over the space between the foils. By a suitable arrangement of foils and spaces (Fig. 3), any desired information can be obtained concerning activity, time of entering and leaving the hiding place, etc.Newts and salamanders need a damp atmosphere. It was found that a moist paper on the bottom of the container will increase the sensitivity if its degree of humidity be kept constant.