Jun 152017

This date in 1752 is the traditional date set for Ben Franklin’s kite flying experiment meant to prove that lightning is electricity. If he had performed the experiment as commonly thought of, and depicted, he would almost certainly have been electrocuted.  He was much more cautious, however, and it’s worth discussing Franklin and electricity on this date because what he discovered led to new directions for science. First, I will admit that I have already discussed this topic briefly here: http://www.bookofdaystales.com/benjamin-franklin/  This is what I said:

According to the canonical tale, Franklin realized the dangers of using conductive rods and instead used a kite. According to the legend, Franklin kept the string of the kite dry at his end to insulate him while the rest of the string was allowed to get wet in the rain to provide conductivity. A house key was attached to the string and connected to a Leyden jar (a primitive capacitor), which Franklin assumed would accumulate electricity from the lightning. The kite wasn’t struck by visible lightning (had it done so, Franklin would almost certainly have been killed) but Franklin did notice that the strings of the kite were repelling each other and deduced that the Leyden jar was being charged. Franklin reportedly received a mild shock by moving his hand near the key afterwards, because as he had estimated, lightning had negatively charged the key and the Leyden jar, proving the electric nature of lightning.

Fearing that the test would fail, or that he would be ridiculed, Franklin took only his son to witness the experiment, and then published the accounts of the test in third person. The standard account of Franklin’s experiment was disputed following an investigation and experiments based on contemporaneous records by science historian Tom Tucker, the results of which were published in 2003. According to Tucker, Franklin never performed the experiment, and the kite as described is incapable of performing its alleged role. Further doubt about the standard account has been cast by an investigation by the television series MythBusters. The team found evidence that Franklin would have received a fatal current through his heart had the event actually occurred. Nevertheless, they confirmed that certain aspects of the experiment were feasible – specifically, the ability of a kite with sufficiently damp string to receive and send to the ground the electrical energy delivered by a lightning strike.

Now let’s look deeper.

Franklin started exploring the phenomenon of electricity in 1746 when he saw some of Archibald Spencer’s lectures using static electricity for illustrations. Franklin proposed that “vitreous” and “resinous” electricity were not different types of “electrical fluid” (as electricity was called then), but the same “fluid” under different pressures. He was the first to label them as positive and negative respectively, and he was the first to discover the principle of conservation of charge. In 1748 he constructed a multiple plate capacitor, that he called an “electrical battery” (not to be confused with Volta’s pile) by placing eleven panes of glass sandwiched between lead plates, suspended with silk cords and connected by wires.

In 1750, he published a proposal for an experiment to prove that lightning is electricity by flying a kite in a storm that appeared capable of becoming a lightning storm. On May 10, 1752, Thomas-François Dalibard of France conducted Franklin’s experiment using a 40-foot-tall (12 m) iron rod instead of a kite, and he extracted electrical sparks from a cloud. On June 15 Franklin may possibly have conducted his well-known kite experiment in Philadelphia, successfully extracting sparks from a cloud, but there is no definitive evidence for this. However we do know that Franklin did conduct kite experiments around this time, although the results were not written up (with credit to Franklin) until Joseph Priestley’s 1767 History and Present Status of Electricity. Franklin was careful to stand on an insulator, keeping dry under a roof to avoid the danger of electric shock. Prof. Georg Wilhelm Richmann replicated the experiment in Russia in the months following Franklin’s experiment and was, indeed, killed by electrocution.

In his writings, Franklin indicates that he was aware of the dangers and offered alternative ways to demonstrate that lightning was electrical, as shown by his use of the concept of electrical ground. If Franklin ever did perform the experiment he proposed he certainly did not do it in the way that is often described—flying the kite and waiting to be struck by lightning. He did however use a kite to collect some electric charge from a storm cloud to prove that lightning was electrical. On October 19 in a letter to England with directions for repeating the experiment, Franklin wrote:

When rain has wet the kite twine so that it can conduct the electric fire freely, you will find it streams out plentifully from the key at the approach of your knuckle, and with this key a phial, or Leyden jar, may be charged: and from electric fire thus obtained spirits may be kindled, and all other electric experiments [may be] performed which are usually done by the help of a rubber glass globe or tube; and therefore the sameness of the electrical matter with that of lightening completely demonstrated.

Franklin’s electrical experiments led to his invention of the lightning rod. He noted that conductors with a sharp rather than a smooth point could discharge silently, and at a far greater distance. He surmised that this could help protect buildings from lightning by attaching “upright Rods of Iron, made sharp as a Needle and gilt to prevent Rusting, and from the Foot of those Rods a Wire down the outside of the Building into the Ground; … Would not these pointed Rods probably draw the Electrical Fire silently out of a Cloud before it came nigh enough to strike, and thereby secure us from that most sudden and terrible Mischief!” Following a series of experiments on Franklin’s own house, lightning rods were installed on the Academy of Philadelphia (later the University of Pennsylvania) and the Pennsylvania State House (later Independence Hall) in 1752.

In recognition of his work with electricity, Franklin received the Royal Society’s Copley Medal in 1753, and in 1756 he became one of the few 18th-century North Americans elected as a Fellow of the Society. He received honorary degrees from Harvard and Yale universities. The cgs unit of electric charge has been named after him: one franklin (Fr) is equal to one statcoulomb.

Franklin advised Harvard University in its acquisition of new electrical laboratory apparatus after the complete loss of its original collection, in a fire which destroyed the original Harvard Hall in 1764. The collection he assembled would later become part of the Harvard Collection of Historical Scientific Instruments, now on public display in its Science Center.

According to Michael Faraday, Franklin’s experiments on the non-conduction of ice are worth mentioning, although the law of the general effect of liquefaction on electrolytes is not attributed to Franklin. However, as reported in 1836 by A. D. Bache of the University of Pennsylvania, the law of the effect of heat on the conduction of bodies that are otherwise non-conductors, for example, glass, could be attributed to Franklin. Franklin writes, “A certain quantity of heat will make some bodies good conductors, that will not otherwise conduct …And water, though naturally a good conductor, will not conduct well when frozen into ice.”

Franklin took a great deal of interest in the food products grown in Britain and North America, generally expressing a preference for the latter.  He did, however, send seeds of kale (which he called “Scotch cabbage”) and rhubarb to friends back home. I suspect he was more interested in the medicinal properties of the roots of rhubarb than the food uses of the stalks. In return he asked his wife to send him apples and cranberries which she sent by the barrel load !! He could get apples in England, of course, but he preferred American Newton pippins for roasting over the apples he could find there. Cranberries of several species are indigenous to all northern temperate regions, but they did not catch on as a domesticated species in Europe in the way that they did in North America. Even in the United States they’re hard to find uncooked these days except around Thanksgiving; most are commercially processed into juice, sauces, and jellies. If you can find plain cranberries you might consider making a tart. Here’s one idea using a custard filling and quite a lot of brown sugar to counter the sourness of the cranberries. I generally use a prepared tart shell out of laziness, but you can make your own pastry if you wish.

Cranberry Custard Tart


9” tart crust
1 ½ cups granulated sugar
¼ cup water
2 cups cranberries (10 oz)
2 large eggs
½ cup brown sugar
1 ½ tsp all-purpose flour
¼ cup light cream
½ tsp almond extract
confectioners’ sugar


Preheat the oven to 350°F/175°C.

Blind bake the tart shell by lining it with foil and filling with pie weights or dried beans. Bake the tart shell for about 30 minutes, until the rim is lightly golden. Remove the foil and weights and bake for another 5 minutes, until it is lightly golden all over. Set the tart pan on a wire rack.

Increase the oven temperature to 375°F/190°C

Meanwhile, make the filling in a medium saucepan by combining the granulated sugar with the water and cook over moderately high heat, stirring, until the sugar dissolves. Add the cranberries, cover and cook over moderate heat for 3 minutes, stirring once or twice. Remove the pan from the heat and let the cranberries cool to room temperature. Drain the cranberries well and reserve the cranberry syrup.

In a medium bowl, make a custard by beating the eggs with the brown sugar and flour. Whisk in the light cream and the almond extract. Spread the cranberries in the tart shell. Drizzle 1 tablespoon of the reserved cranberry syrup over the cranberries, then pour in the almond custard.

Bake the tart in the lower third of the oven until a skewer inserted in the center comes out clean, 16 to 18 minutes. Transfer the tart in the pan to a wire rack to cool completely, at least 2 hours. Dust with confectioners’ sugar.

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