Crack frog
A crackling frog is a spring made from a strip of spring steel . The steel is shaped in such a way that it has a stable and a metastable state. It is bent by the application of force until it suddenly passes through the metastable state due to bumps .
The sudden jump at this point creates the loud cracking noise that gives it its name. When the force decreases again, there is a jump back, which produces another loud crack. Actuated directly at the ear, crackers can generate a sound pressure level of up to 135 dB, which can lead to hearing damage.
Applications
The most obvious use of the cracking frog is as a children's toy, the spring steel is usually combined with a small sheet metal handle in order to be able to easily build up the pressure between thumb and forefinger. The handle is occasionally printed with a frog motif. In so-called rattle boats , a cracking frog generates the characteristic noise, in animal training it is used in clicker training and in heat cushions it triggers the crystallization process through acoustic shock waves , which releases the latent heat. During the Second World War , British infantrymen used the crackling frog as a distinguishing mark in night raid operations.
Mechanisms based on comparable principles are also used in technical applications. Closing and holding mechanisms, however, are mostly bistable , i.e. stable in both states. An example of this are clamps for closing tea bags .
The effect has another application with some twist-off screw caps, which indicate that the glass is unopened and the negative pressure in the glass is intact. Due to the negative pressure, the central part, which is shaped like a cracking frog, is kept in the metastable state, if air flows into the glass, the lid goes into the stable state.
The principle is also often used for hair clips made of sheet metal. Here, however, it is not an impression, but a bending of two arms and their riveting that leads to the two tense states (bistable behavior).
Snap disk
An analog application are pushbuttons , e.g. B. in the control panel of many electronic devices, such as computer mice , which should have a noticeable pressure point. Here, however, the effect is mainly perceived tactilely. The click spring is usually the contact-making component and is called a snap disk .