The Twist Doorbell: A Star Cam, a Pawl and a Spring-Thrown Hammer

"My invention relates to certain new and improved devices for operating the hammer of an alarm or call bell; and it consists in a pawl, in combination with an E-shaped or other suitably bent lever, pivoted to the upper arm of said lever at its end, and so constructed and arranged as to catch and lift one end of the lever to which the hammer is attached, and carry it a sufficient distance to allow the hammer to make its stroke and then release it". "G is a star-shaped cam-wheel". "A square aperture is made through the journal of said wheel for the reception of the key K, by which it is rotated." "U is a stud or stop attached to the base-plate, which limits the stroke of the hammer, and causes the hammer-lever to spring back away from the bell, after the stroke has been made, so that the bell can vibrate freely." "Upon raising the upper arm of the lever D, the pawl lifts the end of the hammer-lever L, and carries it upward until it passes clear of it. Being thus released, the lever is pulled violently back to its original position".

Unscrew the gong of a mechanical twist doorbell. Count the points on the cam and check it by turning the key once slowly and counting strokes. Measure how far the hammer is drawn back with a caliper. Ring it once and time the sound until you can no longer hear it; then hold the hammer against the gong with a finger and strike the gong with a pencil to hear how the touch deadens it. Put it back together without bending the stop, or the hammer will rest on the gong.

**William H. Nichols** of East Hampton, Connecticut, assignor to the Bevin Brothers Manufacturing Company, filed US 153,271 on 29 June 1874; it is dated 21 July 1874. The lever can also be worked by a thumb-stud through a slot in the base. **Honest limits.** The patent gives no sizes or number of cam points. Spring stiffness, stretch, hammer mass, bell frequency and Q in the notebook are examples.