Figure 4-46. - Various types of bimetallic strips.

corrosive to brass. When the temperature around the bellows increases, the heat gasifies the liquid inside and causes the bellows to extend. The extension closes a set of electrical contacts, as shown in figure 4-47, view B. When the bellows cools again, it contracts. The contraction opens the electrical contacts.

Remote-Bulb Device. - Liquid-filled devices are not always limited to the simple bellows. There are some remote-bulb devices that not only have a bellows but also have a capillary tube and a liquid-filled bulb, as shown in figure 4-48.

When the liquid in the bulb is heated, part of it gasifies and forces its way through the capillary tube into the bellows. This increased pressure inside the bellows causes it to extend and open a set of electrical contacts (or open or close a valve). When the bulb cools, the gas liquefies and decreases pressure inside the bell ows. This decreased pressure allows the bellows to contract and close the electrical contacts.

Pressure-responsive devices are actuating mechanisms installed in units, such as steam-pressure controls, steam-pressure gauges, and pressure regulators.

Bellows. - One type of pressure-responsive actuating device uses bellows in a way similar to that of the remote-bulb type. In this application, the bellows extends and contracts in response to changes in steam pressure. The action caused by movement of the bellows opens or closes a set of electrical contacts.

Bourdon Tube. - Another type of pres sure-responsive actuating device is found inside of the pressure gauge shown in figure 4-49. In this actuating device, the pressure is applied inside a

Figure 4-47. - Tension devices: A. Vapor-tension device; B. Pressure-tension device closing electrical contacts.