An occasional piece of apparatus can be traced back to one original paper in the scientific literature. The siren, a device for determining the absolute frequency of a sound, was first described by the French physicist, Baron Charles Cagniard de la Tour (1779-1859) in 1819. All of the sirens on this page follow the original de la Tour design except for the example from St. Patrick's College, which is the design of A. Seebeck from 1840.

   In all sirens puffs of air are cut off by holes in a rotating disk, and the rate at which the puffs of air are formed is the frequency of the sound that is produced. The frequency of sound is controlled by the rotation rate of the disk, and the number of holes cut off in one rotation. The Allegheny, Colby, Miami, Kenyon and Glasgow sirens have one row of holes, while the Hampden-Sydney instrument has several rows of holes and can sound one or more notes at the same time. This improvement is due to H. W. Dove in 1851.

   The disk in the Seebeck design must be driven from an external source, while the de la Tour sirens have the holes in the disk drilled at an angle, which causes the escaping air to drive the disk. The Maynooth siren, while sold by Yeates and Son of Dublin, has a siren disk clearly made by Rudolph Koenig, and listed at 90 francs (about $22.00) in his 1889 catalogue. The Seebeck siren from Vanderbilt is shown in the 1885 Queen catalogue as being imported from Koenig, and was sold for $224.00.

   The de la Tour instruments have geared counters attached to the shaft of the rotating disk. When used with a stopwatch, this feature makes it possible to find the frequency of the sound.

   Sirens must be used with caution in the lecture hall. In the fall of 1964, when trying out a siren for a demonstration, I had the campus safety patrol arrive within seconds!

REFERENCE: Thomas B. Greenslade, Jr., "Apparatus for Natural Philosophy; the Siren", Rittenhouse, 2, 11-13 (1987)
                   Miami University                                     Kenyon College                     St. Patrick's College - Seebeck                              Glasgow University                                                 Vanderbilt University - Seebeck type
 

The front and back of a siren in the Greenslade Collection are shown here. It was made by the "Société Genevoise Pour la Construction D'Instruments de Physique, Genève" and imported into the United States by James G. Biddle of Philadelphia.     The rotating mechanism is light and runs freely, so that a fairly high-pitched sound may be made by blowing into the air inlet hole in the base. One of the indicating hands is missing.

 The ultimate siren is the Helmholtz double siren. Four examples are shown below. Two Dove sirens coupled on the same shaft give a wide variety of frequencies, which all slide up or down the scale as the air pressure increases or decreases.

   Turning the crank at the top rotates the holes in the upper chamber which feed air to the rotating upper siren disk, thus changing the frequency of the sound. With this adjustment, the variation in the speed of rotation of the siren disks, and the ability to open and close each of the sets of holes in the top and bottom siren disks, a number of demonstrations can be performed to illustrate interference and beat phenomena.

   The apparatus at the left, below, is at the museum at the Department of Natural Philosophy of Glasgow University. The placard reads, "Given to Kelvin by H. von Helmholtz..." It was very likely made by Rudolf Koenig of Paris, athough it is not shown in the 1889 Keonig catalogue. I base this statement on the fact that it is illustrated on pg 402 of Zahm's text (see the reference below), and Zahm tended to use apparatus and illustrations provided by Koenig.

   At the right, below is a double siren from Duke University. The apparatus was made by the firm of Max Kohl of Dresden, and cost 300 Marks ($70.00) about 1900. A model driven by a constant-speed electric motor was 550 Marks.
 

Here is a slightly different Max Kohl Double Siren from Case Western Reserve University in Cleveland, Ohio.

Here are the front and back sides of a Helmholtz double siren by Rudolph Koenig at the Uni-    versity of Vermont. It is listed at 540 francs ($90.00) in the 1889 Koenig catalogue. The rubber tubing, visible on the example at the left, above, is missing.