switch contacts are typically rhodium, ruthenium, iridium, or tungsten. There are also versions of reed switches with mercury-wetted contacts. Such switches must be mounted in a particular orientation, lest drops of mercury bridge the contacts even when not activated. Since the contacts of the reed switch are sealed away from the atmosphere, they are protected against atmospheric corrosion. The hermetic sealing of a reed switch make them suitable for use in explosive atmospheres where tiny sparks from conventional switches would constitute a hazard. One important quality of the switch is its sensitivity, the amount of magnetic field necessary to actuate it. Sensitivity is measured in units of ampere-turns (AT), corresponding to the current in a test coil multiplied by the number of turns in the test coil. Typical pull-in sensitivities for commercial devices are in the 10 to 60 AT range. The lower the AT, the more sensitive the reed switch. Smaller reed switches, which have smaller parts, are generally more sensitive to magnetic fields. In production, a metal reed is inserted in each end of a glass tube and the ends of the tube are heated so that they seal around a shank portion on the reeds. Green-colored infrared-absorbing glass is frequently used, so an infrared heat source can concentrate the heat in the small sealing zone of the glass tube. The thermal coefficient of expansion of the glass material and metal parts must be similar to prevent breaking the glass-to-metal seal. The glass used must have a high electrical resistance and must not contain volatile components, such as lead oxide and fluorides, which can contaminate the contacts during the sealing operation. The leads of the switch must be handled carefully to prevent breaking the glass envelope. The glass envelope can be damaged if the reed switch is subjected to mechanical stress. Most reed switches are filled with nitrogen at atmospheric pressure. After the final seal is made, the switch cools and the internal pressure is less than one atmosphere. Reed switches sealed with a pressurized nitrogen atmosphere have a higher breakdown voltage and are useful for switching 220–240 VAC power. Reed switches with a vacuum inside the glass envelope can switch thousands of volts. Reed switches can be used to directly switch a variety of loads ranging from nanovolts to kilovolts, femtoamperes to amperes, and DC to radio frequency. Other magnetically-activated switching devices have a limited range of