The shocking history of capacitors

Like all capa­cit­ors, ours ori­gin­ates from the Ley­den jar, a glass bottle that can store elec­tric­al charge. Ewald Georg von Kleist was the first to exper­i­ence this abil­ity when he received a severe elec­tric shock in Octo­ber 1745. Pieter van Musschen­broek fol­lowed suit when he repeated the exper­i­ment a few months later, in Janu­ary 1746. This is the story about their shock­ing dis­cov­ery and the early devel­op­ment of the capa­cit­or – a  ground­break­ing com­pon­ent that is ubi­quit­ous in today’s electronics.

The early history of electricity

The first known obser­va­tion of what we now call stat­ic elec­tri­city was made by the Greek philo­soph­er Thales of Mile­tus in 600 BC. He noted that amber, when rubbed against cloth, attracts light objects such as hairs. But it took more than two thou­sand years before any­one set out to explore this force.

Sir Wil­li­am Gilbert’s mag­num opus – De Mag­nete,  pub­lished in 1600 – con­tains some of the earli­est sys­tem­at­ic stud­ies of elec­tri­city. He observed that sub­stances like glass, sul­fur, and dia­mond exhib­ited the same attrac­tion prop­erty as amber when rubbed. He called this an elec­tric force. The name is derived from the Greek word for amber.

Six dec­ades later, Otto von Guer­icke inven­ted a simple machine to gen­er­ate stat­ic elec­tri­city. His elec­tro­stat­ic gen­er­at­or con­sisted of a ball of sul­fur cast on an iron axle, which, when pulled around and sub­jec­ted to fric­tion, gave off elec­tric charges, which mani­fes­ted them­selves in sparks.

In 1705, Fran­cis Hauks­bee cre­ated an improved elec­tro­stat­ic gen­er­at­or. It con­sists of a crank that rotates a large wheel from which a belt runs to a smal­ler wheel attached to an axle through a glass ball.

At the begin­ning of the 1730s, Charles François de Cisternay du Fay, also known as just Dufay, dis­covered the exist­ence of two types of elec­tric­al charges. He named them vit­reous and res­in­ous after the mater­i­al he used to pro­duce them (glass and res­in), but we call them pos­it­ive and neg­at­ive. Du Fay also noticed that two of the same repel and two of the oppos­ite attract each oth­er. Moreover, he dif­fer­en­ti­ated mater­i­als in elec­trics and non-elec­trics, which are sim­il­ar but not identic­al to what we today call insu­lat­ors and con­duct­ors.

Igniting alcohol

In the fall of 1745, things sparked the explor­a­tion of electricity.

First up was Mat­thi­as Bose, who makes a name for him­self as a flam­boy­ant demon­strat­or of exper­i­ments with stat­ic elec­tri­city. In one of his most fam­ous tricks, he ignites alco­hol float­ing on top of the water by gen­er­at­ing stat­ic elec­tri­city, which he con­ducts through a met­al bar to the water.

His main con­tri­bu­tion to this story is the use of the met­al bar. He hung it hori­zont­ally with one end above the Hauksbee’s rotat­ing glass ball. If the dis­tance is not too great, or if a met­al chain hangs from the met­al bar down to the glass ball without touch­ing it, the met­al bar will cap­ture stat­ic elec­tri­city that can then be trans­ferred to some­thing else. In Bose’s show, it was the water with alco­hol on the surface.

Zap!

Next up was Ewald Georg von Kleist. Inspirerad av Bose’s met­al bar, he tried to prove that elec­tri­city can be under­stood as a fluid.

On Octo­ber 11, 1745, he filled a small medi­cine bottle with alco­hol and closed it with a cork through which a nail was inser­ted. He then used a met­al bar to trans­fer stat­ic elec­tri­city from an elec­tro­stat­ic gen­er­at­or to the nail. In this way, he ima­gined that stat­ic elec­tri­city was poured into the alcohol.

Von Kleist knew that the glass is an insu­lat­or. There­fore, he was con­vinced that stat­ic elec­tri­city could be “cap­tured” and retained in the bottle.

He acci­dent­ally touched the nail.

Zap!

He was thrown across the room.

Von Kleist had received a strong elec­tric shock, prov­ing that he had cap­tured elec­tri­city in the bottle (but not in the way he thought). In fact, he had cre­ated the world’s first capacitor.

Disappointments

Von Kleist wrote about his exper­i­ment to sev­er­al oth­er elec­tric­al exper­i­ment­al­ists. Some wanted to try it themselves.

Warned by Von Kleist’s example, they kept their dis­tance when the exper­i­ment was repeated. This proved unne­ces­sary, as the exper­i­ments failed, and noth­ing happened.

What a disappointment.

Amateur night

Von Kleist didn’t write to Ander­as Cunaeus, a law­yer and ama­teur sci­ent­ist. Yet Cunaeus came up with some­thing strik­ingly sim­il­ar. Maybe he had heard about Von Kleist’s exper­i­ment. Or not. Nev­er­the­less, he did a sim­il­ar exper­i­ment with house­hold items at his home.

The res­ult?

Zap!

Cunaeus was out for two full days.

Leiden University, anno Domini 1746

It is a cold even­ing in Janu­ary 1746. Snow is fall­ing thickly in the court­yard of Leiden Uni­ver­sity – the old­est in the Neth­er­lands. A few tardy stu­dents are cross­ing the court­yard from a lec­ture hall to the evening’s sup­per. They pass by the labor­at­ory win­dow of Pieter van Musschenbroek.

Pro­fess­or van Musschen­broek stands at the win­dow, think­ing of his friend, the law­yer Ander­as Cunaeus, who has done what pro­fes­sion­als have failed to do – recre­ate von Kleist’s exper­i­ment from three months ago. Now, he will try to repeat the exper­i­ment himself.

He adjusts the chain so that its free end comes as close as pos­sible to the glass ball without touch­ing it.

Mean­while, one of his dis­ciples hangs a met­al wire over the oth­er end of the met­al rod. He then fills a glass jar with water.

They are now ready for the experiment.

Setting up the experiment

While one of the stu­dents cranks the wheels of the elec­tro­stat­ic gen­er­at­or, Pro­fess­or van Musschen­broek takes the water-filled glass jar with his bare hands and holds it up so that the met­al wire is lowered into the water.

Anoth­er stu­dent now takes clothes in his hands and holds them against the rotat­ing glass ball. The fric­tion between the glass and the cloths cre­ates stat­ic elec­tri­city, passing to the met­al bar through the met­al chain and fur­ther to the water through the met­al wire.

While stand­ing there, he thinks about Bose. He may be an assidu­ous self-pro­moter, but stor­ing and trans­fer­ring stat­ic elec­tri­city with a met­al bar was pretty clever.

Prove up

It’s time for the exper­i­ment itself. Pro­fess­or van Musschen­broek reaches out with his left hand to the met­al wire hanging into the glass jar he holds in his bare right hand.

Zap!

Never try again

On Janu­ary 20, 1746, Pro­fess­or Musschen­broek wrote to his des­ig­nated con­tact at the Par­is Academy and told him about the exper­i­ment. He began his letter:

I would like to tell you about a new but ter­rible exper­i­ment, which I advise you nev­er to try your­self, nor would I, who have exper­i­enced it and sur­vived by the grace of God, do it again for all the king­dom of France.

Pieter van Musschen­broek, Janu­ary 20, 1746

Abbé Jean-Ant­oine Nol­let con­firmed the exper­i­ment and then read Musschenbroek’s let­ter at a pub­lic meet­ing of the Par­is Academy in April 1746. He named the elec­tric­al stor­age device Ley­den jar, after Pro­fess­or Musschenbroek’s university.

Grounded

Pro­fess­or van Musschen­broek real­ized that a con­di­tion for the exper­i­ment to suc­ceed was that there was a con­duct­or con­nec­ted to earth on the out­side of the glass jar.

In the cases of von Kleist, Cunaeus, and him­self, they were the con­duct­or to earth, as they held the glass jar with their bare hands.

Those who failed had put down the glass bottle for fear of an elec­tric kiss. (In all hon­esty, they fol­lowed the best prac­tices of their time and delib­er­ately ensured that the glass jar was not grounded).

The novelty is spreading

The Ley­den jar did not only shock Musschen­broek. Soci­ety was also shocked – both lit­er­ally and figuratively.

Self-pro­claimed “elec­tri­cians” held pub­lic demon­stra­tions where they gave spark­ling shows and jol­ted their audi­ence. Nat­ur­al philo­soph­ers elec­tro­cuted anim­als to bet­ter under­stand this new force. Phys­i­cians applied elec­tric shocks to humans to cure vari­ous ail­ments. And tech­no­lo­gists sent charges through wires over rivers and lakes to fig­ure out what it could be used for.

The news of Ley­den jar’s abil­ity to store elec­tric­al charge made its way across the pond to what, for a few more years, would only be referred to as the Amer­ic­an Colon­ies. There, Ben­jamin Frank­lin exper­i­mented with Ley­den jars.

Is water necessary?

It is 1748, and we are at the home of Ben­jamin Frank­lin in Phil­adelphia. He has just filled a Ley­den jar with charge and put it on a glass insulator.

With a look of determ­in­a­tion, he begins his experiment.

Frank­lin pulls out the cork of the jar and lifts it with the wire through it. He grasps the bottle with one hand, and brings a fin­ger of the oth­er hand near its mouth. A strong spark comes from the water, as pain­ful as if he had touched the wire before remov­ing it. It con­vinces Frank­lin that the elec­tric charge is not in the wire. It is still in the jar.

He pours water from the charged jar into an empty second jar. The second jar shows no sign of elec­tric charge. Thus, the elec­tric charge must remain in the now empty first jar.

“What the frock!” Frank­lin exclaims.

He reaches for the teapot con­tain­ing fresh, unelec­tri­fied water and pours new water into the first jar. Test­ing it again, he finds it still cap­able of giv­ing him a jolt.

He later writes:

Thus the whole Force of the Bottle and Power of giv­ing a Shock, is in the Glass itself; the Non-elec­trics in Con­tact with the two Sur­faces serving only to give and receive to and from the sev­er­al Parts of the Glass; that is, to give on one Side, and take away from the other.

Ben­jamin Franklin

Does the shape matter?

Frank­lin now asked wheth­er the shape of the jar is cru­cial to its abil­ity to store charge.

He took a piece of win­dow glass and put it in his hand to test this. On top of it, he then puts a plate of lead that he had electrified.

Now comes the test itself: He puts a fin­ger to the plate. Zap! There was a spark and shock. In oth­er words, the shape doesn’t matter.

Where is the charge stored?

But where is the charge stored? On the glass? Or on the hand and the lead plate in con­tact with the glass?

Frank­lin placed a piece of win­dow glass between two lead plates to find out. The whole stack rests in his hand while he elec­tri­fies the top plate.

He then sep­ar­ates the parts. The glass plate gave off tiny sting­ing sparks when he touched it. He could feel this in many places on the glass sur­face. He also notes that there are no charges in the lead plates. Finally, he returned the glass between the lead plates.

Now, the moment of truth: Frank­lin grabs both lead plates. Zap! A strong jolt showed that the charge was still there.

From this exper­i­ment, Frank­lin con­cludes that the elec­tric charge was on the glass and that the lead plates only served to bring the charge to or from its surface.

Not first

Frank­lin was not the first to dis­cov­er that water is unne­ces­sary and a glass plate works just as well as a glass jar to hold a charge. John Bevis had already demon­strated this in the same year. How­ever, Frank­lin did not find out until later.

But unlike Bevis, who thought that the charge was in the met­al in touch with the glass plate, Frank­lin proved that the charge is actu­ally on the sur­face of the glass.

Positive is lack of negative

Moreover, Frank­lin also figured out that there are not two types of charges, as du Fay had stated almost two dec­ades earli­er, but only one charge: the neg­at­ive one. A pos­it­ive charge arises when a neg­at­ive charge is removed.

In oth­er words, instead of see­ing pos­it­ive and neg­at­ive as two sep­ar­ate entit­ies, Frank­lin viewed them as two states: the pres­ence of a neg­at­ive charge and the absence of it.

With these insights, gained just a few years after the dis­cov­ery of the Ley­den jar, it was now pos­sible to explain what happened on that snowy winter even­ing when Pro­fess­or van Musschen­broek had him­self electrified.

No escape

The fric­tion against the glass ball gen­er­ates pos­it­ive charges. These are passed through the chain, bar, and wire into the water.

Since equal charges repel each oth­er, the pos­it­ive charges are pushed against the inside of the wall of the glass jar. Since the glass is an insu­lat­or, the charges can­not escape the jar.

Redistribution

But the force with which charges repel equal charges and attract oppos­ite charges isn’t stopped by an insu­lat­or. There­fore, the pos­it­ive charges inside the glass jar repel neg­at­ive pos­it­ive out­side the glass and attract neg­at­ive charges.

In its nat­ur­al state, the glass’s exter­i­or has an equal mix of pos­it­ive and neg­at­ive charges. How­ever, the pos­it­ive charges inside the glass repel the extern­al pos­it­ive charges, redis­trib­ut­ing them away from the sur­face while attract­ing neg­at­ive charges closer, con­cen­trat­ing them.

This pro­cess requires a ground path for the dis­placed pos­it­ive charges. This is where the hand becomes cru­cial, act­ing as a con­duct­or to com­plete the cir­cuit and allow these charges to reach the ground through the experimentalist’s body.

Storage

As the pos­it­ive charges in the jar grow, so does the res­ist­ance that newly added pos­it­ive charges must over­come. Even­tu­ally, the jar reaches its max­im­um charge capa­city. There are a large num­ber of pos­it­ive charges on the inside of the glass and an equal num­ber of neg­at­ive charges on the out­side of the glass.

The charges remain as long as there is no way for the pos­it­ive charges to get to the neg­at­ive ones. So you could say that von Kleist was right – the jar stores charge, but not in the liquid, as he thought, but on the out­side and inside of the jar.

But as soon as there is an oppor­tun­ity for the pos­it­ive charges to get to the neg­at­ive ones, they will take it. This is what happened to von Kleist, Cunaeus, and Pro­fess­or van Musschen­broek when they touched the nail or chain that was in con­tact with the water when they held the jar. The pos­it­ive charges rushed through their poor bod­ies – from the hand touch­ing the nail or chain to the hand hold­ing the jar. Zap!

Caveat

The above is a mod­ern descrip­tion of how a Ley­den jar works, using know­ledge from the mid-18th cen­tury. It is a sim­pli­fied view of, in par­tic­u­lar, what hap­pens on the sur­face and inside the glass wall.

It wasn’t until the 1910s that sci­ent­ists had the know­ledge to under­stand what hap­pens at the sub­atom­ic level. It’s pretty damn inter­est­ing stuff, and the story lead­ing up to it is at least as excit­ing as the one we’ve heard so far. But telling this story would take us on too many wind­ing side roads, and dwell­ing on the capacitor’s inner work­ings is anoth­er art­icle, so let’s fast-for­ward the timeline to the begin­ning of the 20th century.

Fast forward

Did you see what I just did? I used the word capa­cit­or in the con­text of the Ley­den jar. That’s because a Ley­den jar is actu­ally a capa­cit­or. That makes von Kleist’s medi­cine bottle the very first ever made.

Fur­ther­more, when Frank­lin put met­al on both sides of a piece of win­dow glass, he cre­ated the world’s first par­al­lel plate capa­cit­or.

In fact, two par­al­lel plates insu­lated from each oth­er are the very essence of a capa­cit­or. The insu­la­tion does not need to be made with glass. It can be vacu­um, air, or any mater­i­al that doesn’t allow charges to move across the gap between the two plates.

Some isol­at­ors, like glass, have the prop­erty that they increase the abil­ity of the capa­cit­or to store charge thanks to a phe­nomen­on called polar­iz­a­tion (sub­ject of anoth­er art­icle). Such an isol­at­or is called a dielec­tric.

All this began to be under­stood in the 19th cen­tury and led to the first mod­ern capacitor.

Birth of the modern capacitor

The Ley­den jar is a high-voltage capa­cit­or. With the devel­op­ment of tele­graphy, tele­phones, and radio in the late 19th cen­tury, there was a need for smal­ler capa­cit­ors for lower voltages. This accel­er­ated the pace of innovation.

The first mod­ern capa­cit­or was developed by D. G. Fitzger­ald. It con­sisted of met­al foil with impreg­nated paper as a dielec­tric. He pat­en­ted the solu­tion in 1876. Paper capa­cit­ors, as they came to be known, were fur­ther developed and widely used through­out the 1950s when plastic film capa­cit­ors began to appear.

Variety of capacitors

The first paper capa­cit­ors were fol­lowed by a for­mid­able explo­sion of dif­fer­ent types of capacitors:

• vari­et­ies of elec­tro­lyt­ic capa­cit­ors, where one plate is replaced by an elec­tro­lyte, includ­ing tan­talum capa­cit­ors and niobi­um capacitors
• mica capa­cit­ors with mica as the dielectric
• ceram­ic capa­cit­ors with a ceram­ic mater­i­al as dielec­tric, includ­ing  ceram­ic disc capa­cit­ors and mul­tilay­er ceram­ic chip (MLCC)  capacitors
• film capa­cit­or with plastic film as dielec­tric, includ­ing PET-capa­cit­ors and PTFE-capa­cit­ors

Capacitors for the future

Cur­rent and future demands for extreme mini­atur­iz­a­tion or for ultra-reli­able and ultra-stable ser­vice res­ult in the devel­op­ment of new types of capacitors.

Integ­rated capa­cit­ors are capa­cit­ors formed by appro­pri­ate metal­liz­a­tion pat­terns on an isol­at­ing sub­strate. These include met­al-oxide-met­al (MOM) capa­cit­ors, met­al-oxide-semi­con­duct­or (MOS) capa­cit­ors, and met­al-insu­lat­or-met­al (MIM) capa­cit­ors. Des­pite the name of these types of capa­cit­ors, they can be encap­su­lated and sold as reg­u­lar, although very tiny, capa­cit­ors. Some­times, they are also called sil­ic­on capa­cit­ors since the sub­strate is usu­ally sil­ic­on. Sil­ic­on com­pounds can also be used as dielectrics.

Deep trench capa­cit­ors (DTCs) are cre­ated on a semi­con­duct­or sub­strate by cre­at­ing deep recesses, called trenches, to max­im­ize the sur­face area and capa­cit­ance in a small foot­print. DTCs are also known as trench sil­ic­on capa­cit­ors (TSC) and sil­ic­on capa­cit­ors (SiCap).

Glass capa­cit­ors are mod­ern Ley­den jars. They con­sist of mul­tiple lay­ers of met­al inter­twined with glass, sim­il­ar to how MLCCs are built. They are used in the most extreme situ­ations. Glass capa­cit­ors are the most dur­able capa­cit­ors in all respects. For example, they can with­stand high doses of nuc­le­ar radi­ation and strong neut­ron radiation.

Note that the term sil­ic­on capa­cit­ors can be used for both integ­rated capa­cit­ors and deep trench capa­cit­ors. Quite confusing.

CNF-MIM capacitors

Of course, we can’t write an art­icle without talk­ing about our car­bon nan­ofiber fiber met­al-insu­lat­or-met­al (CNF-MIM) capacitor.

Without going into detail, we can say that CNF-MIM capa­cit­ors are pro­duced in much the same way as the integ­rated capa­cit­ors of the MIM type. You might have guessed this by the name.

How­ever, they dif­fer from MIM capa­cit­ors in one fun­da­ment­al way where CNF-MIM capa­cit­ors are more sim­il­ar to DTC. Both DTC and CNF-MIM capa­cit­ors use nan­o­tech­no­logy to increase the sur­face area. This is import­ant because the abil­ity to store charges is dir­ectly pro­por­tion­al to the area.

But the area can only increase so much for DTC; the sky’s the lim­it for CNF-MIM capa­cit­ors (almost lit­er­ally). DTCs have trenches that are lim­ited how deep they can go before the sub­strate becomes too brittle and breaks. CNF-MIM capa­cit­ors, on the oth­er hand, build car­bon nan­ofibers on top of the substrate.

Humbling perspective

Pieter van Musschen­broek was not the first to dis­cov­er the Ley­den jar. Still, he was the one who made it known and sparked a flurry of research into the nature of elec­tri­city. Elec­tri­city was stud­ied frantic­ally for the next hun­dred and sixty years or so. It’s an incred­ibly fas­cin­at­ing story, but since we’re approach­ing this article’s end, we’ll leave it at that.

Fast for­ward to today, Smol­tek is driv­ing the devel­op­ment of suc­cessors to the Ley­den jar. The his­tor­ic­al per­spect­ive makes us feel humble. Sure, our CNF-MIM tech­no­logy is a big step for­ward for capa­cit­ors, but it has been pre­ceded by oth­er, more essen­tial steps made by oth­ers before us. We feel happy to be a small link at the end of this 275-year-long chain of dis­cov­ery and development.

Zap!