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Using chargesTHE CARLSONCOPIER

In 1938 American lawyerChester Carlson (1906-1968)

devised a process known aselectrophotography. Hewanted a machine that

could duplicate patentapplication forms -not only the words,but also complicateddrawings. Heinvented an

electrostatic printer, orxerography machine, (fromthe Greekwords xerosmeaning "dry,"and graphos for "writing.") The first xerographic print (above) wasmade by Carlson in 1938, but the first commercial copies were notproduced until the 1950s.

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THE DUST RESTING on atelevision screen is an exampleof electrostatic attraction. Theglass surface of the screenbecomes electrically chargedwhile the television is on. It thenattracts and holds any floatingspecks which happen to comenear. This phenomenon ofelectrostatic forces~wherethere is attraction by unlikecharges, and repulsion by likeones, is put to work in a varietyof modern machines andprocesses. For instance, in thebody-painting shop of a carmanufacturer, tiny droplets ofspray paint are all given thesame electric charge. Theyrepel' each other and areattracted towards the car'sbody, and so settle on it asa more even coating, This

.j,$-exactlythe sameprinciple as the-chargethat amber, when rubbed;produces to pick upfeathers (pp. 6-7)..

,.

Carlson's first print

Early copier (1960)

"DRYWRITING"The early copier uses theattraction of unlike electriccharges.(pp. 10-11), At its heart isa special metal plate coated witha substance called selenium. Apattern of positive charges onthe plate, representing theblack areas to be copied,attracts negatively chargedparticles of a fine blackpowder - the toner.(The toner becomesnegatively charged bycontact with tiny glassbeads in the developingtray.) The toner pattern istransferred to a blank sheet

Developing tray withtoner inside

Making aphotocoPY

Light must bekept out until thedeveloping stageis complete

Selenium-coated plate

with toner adhering to it1CHARGING AND EXPOSING THE PLATE

The selenium-coated plate is put in thecharging chamber and, as the electrified wirespass over, it receives an even coating of positivecharges. The protective shield is replaced and theplate removed to a camera, where it is exposed tothe document to be copied. An image of thedocument is shone on to it by a camera lens.Where light hits the plate, from the white areas

/ of the original document, the selenium becomesa conductor, and the charge flows away. Whereno light reaches, the charge remains.

3THE DEVELOPEDIMAGEThe plate is removed from the

developer tray. This reveals theselenium-coated plate with the finepowder adhering to it, as an exactbut mirror-image replica of theoriginal document. The plate isreturned to the charging chamber.

2DEVELOPING THE PLATE

The plate now has an exact mirror copy of the originaldocument on its sJlrface, in the form of a pattern of positiveelectrostatic charge. With the protective shield inplace theplate is locked to the developing tray. The shiel9. is removed,and the plate swung backward and forward. As this happensthe toner in the developing tray cascades over the plate. Itstiny negatively charged grains are attracted to the positivelycharged areas of the plate, where they stick.

Theplate and paperare slid into the

chargingchamber

The paper nowholds a duplicateof the original

4 POWDERTO PAPERThe next stage also depends on

electrostatic attraction. A sheet of blankpaper is placed over the plate and its powderimage. Theplate and paper are pushed backinto the charging chamber and withdrawn againwhile the transfer switch is depressed. The paperbecomes positively charged so that it attracts thetoner powder away from the plate.

6HEAT-SEALINGTHE COpy

Finally, the plain paper with its powderpattern is placed on the fuser tray andpushed into an oven-like chamber for a fewseconds. The powder bakes and melts intothe fibers of the paper, permanently sealingthe image. The entire process takes aboutthree minutes - much longer than thecouple of seconds in a modern photocopier.

5 REMOVING THE COpy

The paper is carefully lifted from theplate, bringing with it the pattern of toner,which is now stuck to it by electrostaticattraction. The copied parts have beenreversed again, so that they are an exactduplicate of the original.

How an electrostatic precipitator worksDUST-EATINGAIR FILTERThe electrostatic air filter on this 1930s cigarettecard, showing a man demonstrating the f!lterwith cigarette smoke, uses a fan to draw in astream of dusty, impure air. A prefilter trapsthe bigger floating particles. Theremainingsmall ones pass through the first electrifiedgrid of a device called the electrostaticprecipitator. This grid gives each particle anegative charge. The particles are repelledfrom the negative wires onto the.precipitator's second, positively chargedgrid. They are attracted to it, and stick toits mesh. A filter then absorbs any odorsand cleaned air blows out the other end.

Positively charged mesh

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