Ammeter, Ayrton's

A direct reading instrument for measuring current intensity. A solenoid receives the current. In the axis of the solenoid an iron tube is suspended by a long spiral spring that pa es down within it, and the upper end of which spring is fastened to the gla top of the instrument. The tube is provided with proper guides so as to maintain a vertical position, and is free to rotate. Its upper end carries an index. The whole operates as a magnifying device. A slight longitudinal displacement of the tube causes it to rotate through a considerable angle by the action of the spring. By properly proportioning the parts, the angle of displacement of the index is directly proportional to the current between 15º and 270º angular displacement. The same instrument is wound for use as a volt-meter. Its principal fault is its restricted range.