A height indicator for aircraft is described in which a change in capacitance with altitude is measured at an audio frequency. The capacitance change is indicated on a meter, which is calibrated in feet. Two insulated metal electrodes are mounted on the aircraft, and the direct capacitance between them is measured. This capacitance is affected by the earth, which behaves as a conducting sheet, and the variations can be used to determine the altitude of the aircraft. The changes in capacitance are minute, and measurement is made difficult by the presence in the system of other and much larger capacitances. The difficulties can be overcome by using the double-ratio a.c. bridge described in the associated paper, and the sensitivity can be made sufficient to enable capacitance changes of 1 μμμF to be detected. AS the altitude of the aircraft increases, the capacitance changes caused by the earth become ever smaller, and are eventually obscured by changes arising from other causes. The method is, therefore, suitable only for measuring low altitudes; but, within its range, it is capable of some accuracy.