Description: Electric bell.

Caption: The bell has an electromagnet made from two coils of copper wound round two iron bar. When electricity passes through the coils the magnet is energized and attracts a small steel leaf spring to which the bell clapper is attached. As it does so the electrical contact to the magnet is broken and the clapper recoils and strikes the bell. The electrical contact is then re-established and the process repeats itself giving a continuous ring.

Introduction: This electric bell was used with a vacuum pump, a vacuum platen (plate) and a glass bell-jar in class demonstrations of acoustics - see GLAHM 105235 and 105237. Electric bells of this type are based on the switching device, commutator or induction coil interrupter that was designed by Neef. Neef's "hammer" is a type of device that is now called a relay.

Background: Sound is created by vibrations that act on the atoms or molecules of the medium through which it travels. The atoms or molecule move up and down a very small distance but this causes the sound to pass through the medium like the waves on the surface of the sea. The speed of a sound wave is dependent on the density of the medium through which it travels.

Historical Context: Invention of the working principle in Neef's "hammer" - When: 1838 - Where: Frankfurt-am-Main, Germany - Who: Neef, Christian Ernst (1782-1849)

Historical Context: Manufacture - When: circa 1895 - Where: Unknown, the instrument is unsigned

Historical Context: Used at the Gilbert Scott in lecture demonstrations in the Department of Natural Philosophy now called the Department of Physics and Astronomy - Who: Thomson, William, Professor (1824-1907) Sir William Thomson from 1866 and Ist Baron Kelvin of Largs from 1892 better known as Lord Kelvin

Historical Context: Used in lecture demonstrations in the Department of Natural Philosophy now called the Department of Physics and Astronomy - Where: University Avenue, Glasgow, Scotland

Historical Context: Used in the Gilbert Scott B'lding by Prof. William Thomson in the Department of Natural Philosophy now called the Department of Physics and Astronomy - When: circa 1895

Historical Context: Event - "Lord Kelvin: A life in science" exhibition, Hunterian Museum, University of Glasgow. Object on display in this exhibition.