Ampere-Turns (AT) vs milliTesia

A Comparison of the Measured Magnetic Field Strength Using Ampere-Turns (AT) and milliTesla (mT)

With the advent of the Reed Switch, developed by Bell Labs in the 1940s, it was convenient to measure its operate characteristics using the units of ampere turns. Since the Reed Switch is cylindrical it is easy to make the measurement of its closure, release and contact resistance using a coil with a given geometry, wire size and number of turns. It is easy to conventionalize this approach as long as other users, internal or external, find no problem using ampere turns (AT) as their unit of measure.

However, a real problem arises when one finds that no convention has ever been adopted in the Reed Switches' long history; in fact, most manufacturers of Reed Switches have their own standard. Therefore, companies who purchase their Reed Switches for making Reed Relays, Reed Sensors or other reed products find they have to deal with an assortment of AT standards. No true standard is offered to customers who use Reed Relays, Reed Sensors, etc.

Users find themselves selecting reed products with no idea how to categorize or select them for their own applications. This results in much time lost and frustration in trying to select the proper product. Often times, many thousands of dollars may be lost through high production failures or production line shut down, before determining the correct Reed Switch sensitivity selection.

What we plan to present here is a standard that manufacturers of Reed Switches, manufacturers of reed products, and users of reed products can all use. We will present a simple way to bridge the approach of measuring the magnetic field strength of a Reed Switch from the Reed Switch manufacturer/reed product manufacturer to the reed user's application .

Before we present this approach, we need to review a few very important points that generally affect Reed Switch applications.

• When a Reed Switch is initially measured, it is made with its given overall length. This length is established by the manufacturer to offer the users the most flexibility for short and long length design requirements. As one cuts the Reed Switch to a given size for a given application the AT for that switch will change. If now measured in the same coil to a given cut length, the AT will be different. If significant lead length is cut off the AT change can be dramatic. This occurs because the reed blades are ferromagnetic and the more magnetic material present the more efficient the magnetic field strength. Cutting away the magnetic material will reduce the magnetic field strength; thereby, reducing the magnetic sensitivity of the Reed Switch. Some companies for a given special requirement will supply the AT difference in their specification for a given cut length. However, if the user cannot measure his application in the standard test coil used by the Reed Switch supplier because his application does not "fit" into it, which is most times the case, it becomes impossible to directly correlate between the two companies when using AT only.

• Reed Switches that are not cut, but bent into a new configuration, will often undergo an AT change as well. Here, whenever the magnetic path is altered, the magnetic field strength may change depending upon the new given configuration.

• When a Reed Switch is bent into a new configuration with or without cutting the lead length, the AT may be additionally altered by improperly bending the Reed Switch. All Reed Switches have some susceptibility to any stress placed on either end of its glass to metal seal. Some switches are more susceptible than others. In any case, a stress to the seal can alter the mechanical operation and thereby alter its AT. The Reed Switch gap generally averages less than 25 microns (0.001"). Any small mechanical change produced by either a torsional, rotational or linear force can give rise to an AT or contact resistance change. The contact gap, contact design, blade overlap, lead material, lead material hardness, lead material length and thickness, seal strength, seal length, glass length and measurement approach, will all influence the AT of a Reed Switch.

Since the user in most cases can not measure his magnetic field requirements in AT, the easiest way and more accepted way is to measure the requirement in Gauss or milliTesla (mT). Here 10 Gauss is equal to 1 mT making the interchange between the two units an easy task. More generally accepted outside the Reed Switch and reed product manufacturing arena is the use of Gauss and Tesla or milliTesla (mT).

Downloads:

Ampere-Turns (AT) vs milliTesia