Editing Varistor (section)

==History==
The development of the varistor, in form of a new type of [[rectifier]] based on a [[cuprous oxide]] (Cu<sub>2</sub>O) layer on copper, originated in the work by L.O. Grondahl and P.H. Geiger in 1927.<ref>{{Cite journal| date=February 1927 |pages= 357–366|doi=10.1109/JAIEE.1927.6534186|title=A new electronic rectifier|journal=Journal of the A.I.E.E.|volume=46|issue=3|last1=Grondahl|first1=L. O.|last2=Geiger|first2=P. H.|s2cid= 51645117}}</ref>

The copper-oxide varistor exhibited a varying resistance in dependence on the polarity and magnitude of applied voltage.<ref name=att1938>American Telephone & Telegraph; C.F. Myers, L.S.c Crosboy (''eds.''); ''Principles of Electricity applied to Telephone and Telegraph Work'', New York City (November 1938), p.58, 257</ref> It was constructed from a small copper disk, on one side of which, a layer of cuprous oxide was formed. This arrangement provides low resistance to current flowing from the semiconducting oxide to the copper side, but a high resistance to current in the opposite direction, with the instantaneous resistance varying continuously with the voltage applied.

In the 1930s, small multiple-varistor assemblies of a maximum dimension of less than one inch and apparently indefinite useful lifetime found application in replacing bulky electron tube circuits as modulators and demodulators in [[carrier system|carrier current systems]] for telephonic transmission.<ref name=att1938 />

Other applications for varistors in the telephone plant included protection of circuits from voltage spikes and noise, as well as click suppression on receiver (''ear-piece'') elements to protect users' ears from popping noises when switching circuits. These varistors were constructed by layering an even number of rectifier disks in a stack and connecting the terminal ends and the center in an anti-parallel configuration, as shown in the photo of a [[Western Electric]] Type 3B varistor of June 1952 (below).

<gallery>
File:Western Electric Type 3B varistor manufactured in June 1952.jpg|Western Electric 3B varistor made in 1952 for use as click suppressor in telephone sets
File:Varistor circuit historical construction.png|Circuit of the traditional construction of varistors used as click suppressors in telephony<ref>Automatic Electric Co., Bulletin 519, ''Type 47 Monophone'' (Chicago, 1953)</ref>
File:Diac.svg|Traditional varistor schematic symbol,<ref>American National Standard,''Graphic Symbols for Electrical and Electronics Diagrams'', ANSI Y32.2-1975 p.27</ref> used today for the [[diac]]. It expresses the diode-like behavior in both directions of current flow.
File:Western Electric Type 44A varistor on U1 receiver.jpg|Western Electric Type 44A varistor for click suppression, mounted on a U1 telephone receiver element manufactured in 1958.
</gallery>

The [[model 500 telephone|Western Electric type 500 telephone set]] of 1949 introduced a dynamic loop equalization circuit using varistors that shunted relatively high levels of loop current on short central office loops to adjust the transmission and receiving signal levels automatically. On long loops, the varistors maintained a relatively high resistance and did not alter the signals significantly.<ref>AT&T Bell Laboratories, Technical Staff, R.F. Rey (ed.) ''Engineering and Operations in the Bell System'', 2nd edition, Murray Hill (1983), p467</ref>

Another type of varistor was made from [[silicon carbide]] (SiC) by R. O. Grisdale in the early 1930s. It was used to guard telephone lines from lightning.<ref>R.O. Grisdale, ''Silicon Carbide Varistors'', Bell Laboratories Record 19 (October 1940), pp.46–51.</ref>

In the early 1970s, Japanese researchers recognized the semiconducting electronic properties of [[zinc oxide]] (ZnO) as being useful as a new varistor type in a [[ceramic]] sintering process, which exhibited a voltage-current function similar to that of a pair of back-to-back [[Zener diode]]s.<ref>M. Matsuoka, Jpn. J. Appl. Phys., 10, 736 (1971).</ref><ref>Levinson L, Philip H.R., ''Zinc oxide Varistors—A Review'', American Ceramic Society Bulletin 65(4), 639 (1986).</ref> This type of device became the preferred method for protecting circuits from power surges and other destructive electric disturbances, and became known generally as the metal-oxide varistor (MOV). The randomness of orientation of ZnO grains in the bulk of this material provided the same voltage-current characteristics for both directions of current flow.