September 1965 Electronics World
Wax nostalgic about and learn from the history of early electronics. See articles
Electronics World, published May 1959
- December 1971. All copyrights hereby acknowledged.
Reed switches have
been in use for a long time, and are still common today. The position-sensing
ones with mercury inside as the connection making and breaking medium have long
been gone from the market. Mercury is on the boogeyman list of items that shall
not be used in any form, regardless of how inaccessible or how small the amount
happens to be. Mercury is an excellent choice for the job because it
provides a reliable contact without excessive arcing to spoil the contacts, and the angle of make/break is
highly repeatable (especially as compared to a bimetal springs with hard
contacts). Mercury switches were the de facto standard in wall switches for
lighting for decades. Not only do I remember my elementary school teacher
passing beads of mercury around for us kids to experience the properties of
(heavy for its size, liquid metal at room temperature, why it's called
"quicksilver," etc.), but I also remember
breaking apart light switches to get the mercury out of them just to play with.
Drop the bead on the floor and it "explodes" into many tiny balls, which
magically recombine when you touch them together. I'd guess the
danger of such a
small exposure to mercury people were exposed to back in the day pales in
comparison to all the toxic ingredients in food (processing and preservatives),
off-the-shelf common chemicals (i.e., RoundUp™, bug poisons, etc.), the
plethora of "recreational" drugs, and government-mandated virus injections a large portion of society
avails itself of today. This 1965 Electronics World magazine article
has, among other good information, an interesting table comparing data on
typical conventional relays, reed relays, and transistor switching circuits with
approximately the some amount of load-handling capabilities.
Magnetic Reed Switches and Relays
By Gary A. Lehmann
Characteristics of these fairly new components make them suitable for a variety
of electronic applications. Though not as fast as transistor switches, they have
lower contact resistance, higher open-circuit resistance, will switch higher voltages,
and have less capacitance. Practical uses include: tach pickups, proximity switches,
Not too long ago, a new component made its first appearance in electrical and
electronic equipment. As with most really basic inventions, the operation of the
magnetic reed switch is strikingly simple. Rapid acceptance throughout the industry
shows that it filled a definite need for bridging the gap between the ordinary electromechanical
contact or or relay and the solid-state switch.
A survey of conventional relays shows that only specially designed types will
operate faster than four milliseconds, will exhibit low contact bounce and contact
capacitance, will work in a wide range of environment, and have long life expectancy.
The reed relay makes all these features possible, and for a price that is well below
that of the highly specialized electro-mechanical relay.
The reed switch has definite advantages over the transistor switch: high ratio
of open-contact to closed-contact resistance, (typically 1010 to 1),
high breakdown voltage (300 volts at 60 cps and more), and contact capacitance of
less than 1 pf. These properties make the reed switch well suited for use in high-impedance
circuits, to handle higher voltages than a transistor switch, and to switch radio-frequency
Transistor-switching circuits with equal load capacity, while capable of operating
in the microsecond range, seldom exceed "off-on" resistance ratios of 100,000 to
1; collector-to-emitter breakdown voltages of 50 volts to 60 volts; and generally
have base-to-collector and collector-to-emitter capacitances of 40 pf. and 80 pf.
respectively. Unless protected by breakdown diodes, transistors can be damaged quite
easily by voltage transients which will not affect reed switches. Furthermore, a
simple transistor switching circuit provides little insulation between the controlling
and the controlled signal, both are applied to the same transistor which is not
a unilateral device such as the vacuum tube. If the reed switch is actuated by a
coil (reed relay), both signals are electrically insulated from each other and mutual
capacitance between coil and reeds can be low.
Table 1 compares pertinent data on typical electromechanical, reed, and solid-state
relays with similar load capacities. All values given in the table are approximate.
The Contact Capsule
Fig. 1 - An s.p.s.t. normally open magnetic reed switch.
Table 1 - Comparative data on typical conventional relays, reed
relays, and transistor switching circuits with approximately the some amount of
Fig. 2 - Use of a reed switch in tachometer or rpm counter.
The reed switch consists of two or more metal reeds which are enclosed in a hermetically
sealed glass capsule. The reeds are made from nickel-iron, a magnetically "soft"
alloy that retains only a little magnetism. Fig. 1 shows a s.p.s.t., normally open
reed switch. The overlapping ends of the nickel-iron reeds are the contact surfaces
which are usually plated with gold or other suitable noble metal. This will promote
lower contact resistance and help keep the surfaces electrically clean. A chemically
inert gas which is enclosed in the capsule further contributes to the maintenance
of good contact properties. As with any switch, contact arcing should be suppressed
by suitable circuitry; whatever remains is confined to the glass capsule. This permits
operation of the reed switch in chemically active or explosive atmospheres. It can
be seen that the overlapping contact ends of the reed switch represent little mutually
opposing area, hence the inherent contact capacitance is low.
Contact capsules are made in various sizes and for different current- and voltage-switching
ranges, giving the circuit designer a wider range of specifications from which to
choose. Reed switches can be actuated by permanent magnetic or electromagnetic fields,
or by a combination of both.
When a magnet is moved close to a reed switch, more and more of its field lines
tend to permeate the reeds because of their lower magnetic reluctance in comparison
to air. As the magnet moves closer to the reeds, it reaches a point where the mutual
attraction between the reeds begins to pull them toward each other. As the mutual
distance is shrinking, the magnetic flux across the gap between the reeds increases
as the square of the distance of separation. Even if the external magnet were not
moved much beyond the point where mutual attraction begins to pull the reeds together,
the increasing attraction between the reeds causes them to accelerate until they
The reverse action takes place when the external magnet is removed. As the distance
between the magnet and the reeds is increased, the magnetic remanence tends to keep
the ends of the reed together. But since the reeds consist of a magnetically "soft"
alloy, the point is soon reached where the decreasing flux from the receding magnet
is insufficient to hold the reeds together and they begin to separate. At this moment,
the air gap reduces the remaining flux sharply, causing the reeds to recede with
It can be seen that this magnetic effect also helps to reduce contact bounce
which tends to occur, especially at the moment of closure. As the reeds make contact,
the magnetic flux and the mutual attraction increase greatly and help to counteract
the tendency of the reeds to rebound. The magnetic effect, in conjunction with the
relatively low mass of the reeds, permit the reed switch to follow rapid changes
of the actuating magnetic field, allowing switching rates of several hundred cycles
These properties can be put to use, for instance, in a pulse generator, tachometer,
or revolution-counting circuit, as shown in Fig. 2. The drive shaft of the electric
motor rotates a disk made from a magnetic shielding material such as Mu-metal with
a cut-out along part of its periphery. The disk normally shields the contact capsule
from the permanent magnet. As the disk is rotated, the magnetomotive force of the
magnet actuates the contact capsule every time the cut-out permits the flux to reach
the reeds. The reeds are connected in series with a battery and a suitable resistor.
The pulsating current or voltage across the resistor is available for indicating
or processing in external equipment, such as an integrating voltmeter for a tachometer
or an electric counter.
Another application of the contact capsule would be to monitor the condition
of a window or door. The capsule and actuating magnet can be easily concealed in
adjacent parts of the wooden structures as shown in Fig. 3. With the window closed,
the magnet energizes the reeds and thereby maintains a short-circuit across the
pilot light at a guard's desk, preventing the bulb from glowing. As soon as the
window or door is opened, the magnet no longer holds the reeds together, the short-circuit
is removed, and the pilot light is energized. One advantage of the reed switch in
this application is that the invisible magnetic flux actuates the contacts rather
than a stud or contact button which might, by its presence, reveal the presence
of an alarm system on the premises.
Fig. 3 - When the window is opened, the lamp is turned on.
Fig. 4 - Use of magnetic biasing to produce s.p.d.t. switch.
The magnetic reed switch can be actuated by an external permanent magnet as well
as by the magnetic field of current flowing through a solenoid, or by both. When
a small permanent magnet is placed in the vicinity of the reed capsule, the resulting
flux permeates the reeds and acts as a magnetic bias which can be modulated by a
changing magnetic field from the solenoid. With an actuating current of a given
intensity, pull-in or drop-out points of the reed relay can be adjusted by varying
the distance between the external magnet and the contact capsule and coil assembly.
Magnetic biasing makes it possible to convert a normally open reed switch into a
normally closed type. The constant magnetic field keeps the reeds closed. To open
the contact, the electromagnetic flux of the solenoid must counteract the permanent
flux of the mag-. et to neutralize the mutual attraction of the reeds.
Fig. 4 shows how magnetic biasing can be used to make a single-pole, double-throw
switch. The small permanent magnet between the two reeds shown to the right in the
diagram establishes a field with lines of force perpendicular to the main axis of
the contact capsule. Depending on the strength and polarity of the actuating magnetic
field, the large reed is attracted by either the upper or lower contact reed.
For certain applications, even the low contact bounce of a reed switch cannot
be tolerated. This residual bounce may be eliminated by the use of mercury-wetted
contacts. Fig. 5 illustrates this special form of reed switch. A small amount of
pure mercury is enclosed in the contact capsule. The form of the reeds promotes
capillary action and keeps the contact ends of the reeds covered with a thin film
Mercury has a very high surface tension (which accounts for its tendency to form
small globules when spilled on a flat surface). When an object is pressed against
a mercury surface, the mercury recedes until the pressure of the object exceeds
the surface tension of the liquid metal. At this point the object penetrates suddenly
and rapidly and the mercury flows up on the object, seeking to cover as much of
it as possible. When the object is withdrawn, the opposite action takes place. A
mercury filament is formed which is suddenly broken when the surface tension is
exceeded. The same surface tension is responsible for the capillary action which
maintains a constant film at the contact surface that is fed by means of the mercury
pool at the bottom of the capsule.
One disadvantage of the mercury-wetted contact is the need to prevent the mercury
from flowing across the contact gap. To accomplish this, the capsule's longitudinal
axis must be maintained in a perpendicular or near-perpendicular position, with
the mercury pool at its lowest point. This might preclude the use of the switch
in certain types of aircraft or for missiles whose attitude and acceleration in
flight may frequently nullify the action of gravity.
The Reed Relay
The contact capsule can be actuated by a permanent magnet (to form a proximity
switch) or by current flowing through a solenoid to provide the magnetic flux. In
the latter case the arrangement is called a reed relay. Conventional relays require
magnetic forces on the order of ounces, whereas reed relays can be actuated by considerably
less than one ounce. Consequently, reed relays require driving power from as little
as 20 mw. for miniature relays to approximately 500 mw. or more for multiple contact
relays where several contact capsules are actuated by the same relay coil. In a
conventional relay, the driving member is usually spring loaded and mechanically
linked with the contact member. It can be seen from Fig. 6 that the driving member
and contact member in a reed relay are one and the same and that the natural elasticity
of the cantilever construction makes additional spring loading unnecessary.
Table 2 - Characteristics and performance data on a number of
typical magnetic reed switches and relays.
Typical magnetic reed switches. (A) S.p.s.t. standard, 15 va., 500 v., Gordos
Corp. (B) S.p.s.t. miniature, 4 va., 300 v., Gordos Corp. (C) S.p.s.t. miniature
mercury-wetted magnetic reed switch inserted into test coil, 3 va., 100 v. Hamlin,
Fig. 5 - By means of capillary action, the film of mercury from
the pool at the bottom of the capsule covers the contact surfaces. This is done
to prevent contact bounce. The switch must be kept in a vertical position, however,
in order to keep the mercury from bridging the contact gap.
Fig. 6 - Cross-sectional view of a typical magnetic reed relay.
Typical reed relays. (A MIL-type sealed s.p.d.t., C.P. Clare & Co. (B) Printed-circuit
s.p.s.t., normally open, and s.p.s.t., normally closed, Wheelock Signals Inc. (C)
Printed-circuit encapsulated MIL-type, s.p.s.t., manufactured by General Reed Co.
Physical dimensions, position, and magnetic properties of the reeds of a contact
capsule are closely controlled during manufacture. The fabrication of capsules is
largely automatic. This results in remarkable uniformity among one type, as far
as pull-in, drop-out, and contact resistances are concerned. Pull-in and drop-out
points are described in terms of magnetomotive force F which is measured in ampere-turns
A typical contact capsule requires 60 ± 10 ampere-turns for closure and
opens at a flux corresponding to 22±10 ampere-turns. The difference between
pull-in and drop-out results from mechanical and magnetic properties of the reeds,
mainly from the hysteresis of the nickel-iron alloy. Once the working gap of the
reed switch is closed, reluctance of the magnetic circuit is reduced and less magnetomotive
force is required to maintain the flux needed to keep the reeds together. Contact
resistance is on the order of 20-50 milliohms. Operating time is composed of field
development time and reed motion time, and can be reduced by increasing the magnetomotive
force, or electrical power, if a coil is used to activate the reeds. However, there
is a minimum switching time for a given contact capsule, and any further increase
in coil power generally results in an increase in contact bounce only.
To a great extent, the life expectancy of reed relays depends, among other factors,
on the magnitude and type of the switching load. If used in so-called "dry circuits,"
i.e., with no or very low current loads (on the order of 10 ma. at less than 12
volts), billions of operations can be expected. Manufacturers' test reports show
that a typical contact capsule can handle 20 million switching cycles of a 15-watt
non-inductive load. Table 2 indicates the wide range of performance characteristics
of contact capsules and reed relays. The photographs show typical reed relays and
reed switch capsules.
If more than one contact is to be actuated by the same signal, the required number
of contact capsules may be inserted in a common driving coil. In this manner, single-pole,
single-throw and single-pole, double-throw capsules may be combined into one relay.
Power requirements will increase accordingly because more area must be permeated
by the magnetic flux. A minor problem is that, unless the capsules are specially
matched, not all capsules will be actuated at exactly the same time. This particular
characteristic is due to pull-in tolerances among individual reeds.
Reed Relay as D. C. Chopper
Table 2 shows that most reed relays will operate within one millisecond and release
even faster. Contact bounce is in the neighborhood of 10% of the actuating time.
These properties indicate that reed relays may be used as inexpensive d.c. choppers.
Choppers have been around for a long time, the best known is the car radio vibrator,
which was widely used before the advent of transistorized car radios. Precision
choppers are used in commercial and military equipment, such as voltage comparators
and test instruments, where the cost of the precision chopper is of secondary importance.
The attractive feature of a chopper is that it converts a d.c. voltage which can
be measured quite accurately by a good meter movement, into a rectangular waveform
with corresponding peak voltage. The waveform can be handled by a.c.-coupled amplifiers
and also conveniently displayed on an oscilloscope.
The main advantage of an electromechanical chopper over a transistorized square-wave
generator lies in the ease of calibration and the near constant voltage output which
is almost independent of operating temperatures. The base-to-emitter and collector-to-base
voltage drop of transistors vary considerably with temperature, and inexpensive
transistors of the grown-junction type have objectionable base charge characteristics
which preclude saturated operation if a perfect square wave is to be obtained.
The low cost of a reed relay makes it very attractive for use as a d.c. chopper.
Switching rates of several hundred cycles per second are possible, and the high
resistance differential between closed and open condition, together with a lack
of temperature problems, offer considerable advantage over a transistorized switching
The development of the magnetic reed switch filled a definite need for a component
that can supplement the solid-state switch in certain specific areas. Its tolerance
to voltage transients, simple and reliable construction, together with its low cost,
are added advantages which leads to various applications. The imaginative reader
will think of many other uses for this extremely versatile new component.
Reed Relay Manufacturers
136 W. Greenfield /live., Milwaukee 4, Wis.
Automatic Electric Co.
Sub., General Telephone & Electronics Northlake,
Computer Components Inc.
8806 Van Wyck. Expwy., Jamaica 18, N.Y.
Coto-Coil Comp., Inc.
Clare, C.P. Co.
3103 Pratt Blvd., Chicago 45, Ill.
Davis Electric Co.
230 N. Spring Ave., Cape Girardeau, Mo.
Sub., Walter Kidde & Co., Inc.
6 Pawcatuck Ave.,
65 Daly Rd., East Northport, L.I., N.Y.
General Electric Co.
General Purpose Control Dept. Bloomington, Ill.
General Reed Co.
174 Main St., Metuchen, N.J.
250 Glenwood Ave., Bloomfield, N.J.
Grigsby Co., Inc.
407 N. Salem Ave., Arlington Heights, Ill.
Lake & Grove Sts., Lake Mills, Wis.
Hathaway Instruments, Inc.
5800 East Jewell Ave., Denver 22, Colo.
Jaidinger Mfg. Co., Inc.
1921 W. Hubbard St., Chicago 22, Ill.
Line Electric Co.
249 River St., Orange, N.J.
Magnecraft Electric Co.
5575 N. Lynch Ave., Chicago 30, Ill.
Milwaukee Relays, Inc.
Box 123, Cedarburg, Wis.
Minneapolis-Honeywell Regulator Co.
Microswitch Div., Freeport, Ill.
MKC Electronics Corp.
454 E. Donavan Rd., Kansas City 15, Kans.
Radio Corporation of America
Electron Tube Division
R.B.M. Controls Div.
131 Godfrey St., Logansport, Ind.
2908 Nebraska Ave., Santa Monica, Calif.
Revere Corp. of America
845 N. Colony Rd., Wallingford, Conn.
123 W. 155th St., Gardena, Calif.
Lamb's Rd., Pitman, N.J.
Wabash Magnetics, Inc.
Box 454, Wabash, Ind.
Western Electric Co.
Wheelock Signals, Inc.
273 Branchport Ave., Long Branch, N.J.
1132 S. Prairie Ave., Hawthorne, Calif.
Posted August 12, 2022