Franklin’s Experiments

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Franklin’s Experiments

Electricity. What to us is an everyday phenomenon with which we heat our homes, start our cars, light our rooms, and operate our appliances was to colonial Americans a mysterious, unknown force. For centuries Europeans had known about the static charge that results when two objects rub together. Scientists invented a primitive generator in the 1600s and a battery, the Leiden jar, in the next century. But what kind of force was electricity and to what use could it be put were unanswered questions. During the eighteenth century in Europe electricity became the most popular scientific study, the source of entertainment at royal palaces, and the means by which seemingly magical tricks could be performed. Benjamin Franklin’s interest in electricity originated when he saw an itinerant scientific lecturer, Archibald Spencer, perform an “electricity show” in Boston. Soon Franklin acquired enough glass tubes, iron rods, silk, cork, and chains to perform his own experiments. By the late 1740s Franklin spent most of his time performing experiments in electricity and recording his results in various letters to American and European correspondents.

The Kite. Franklin made several important discoveries about electricity. Contemporary European theories suggested that electricity consisted of two fluids, but Franklin found it to be a single force. He realized that this force was present in nature in varying amounts, that its “particles” subtly penetrated matter, and that a net increase of electric charge in one body corresponded with a net decrease of electric charge in another. His most famous discovery confirmed what Europeans had long suspected, that lightning was an electrical phenomenon. In June 1752 Franklin constructed a silk kite with a metal wire protruding from its top. He flew the kite in a thunderstorm while standing in a shed for protection. In his hand he held twine tied to the kite. He tied a silk ribbon to the twine near his hand and attached a key as well. A bolt of lightning never struck the kite. Rather the kite conducted the electric charge of the clouds along the twine to the key. When Franklin moved his hand to the

key, he felt a sharp electric spark. Franklin concluded in a letter to Peter Collinson, “from Electric Fire thus obtained,... all the Other Electrical Experiments [may] be performed, which are usually done by the help of a rubbed Glass Globe or Tube, & thereby the Sameness of the Electric Matter with that of Lightning compleatly demonstrated.”

THE FRANKLIN STOVE

When the venerable Franklin stove is mentioned, one invariably thinks of a large cast-iron stove with a simmering kettle. But the Franklin stove of popular imagination is quite different from the one of history. Franklin described and advertised his invention in a 1744 pamphlet, An Account of the New Invented Pennsylvanian Fire-Places. The purpose of the fireplace, or stove, was to warm cool air efficiently and displace it into a room while safely forcing smoke out of a flue. Franklin’s pamphlet included a diagram of the fireplace, which was made entirely of iron. Cool air entered through a hole in the bottom plate. The cool air warmed in the air box, which was heated by wood burning in the fireplace. Heated air entered the room by means of ventilated side plates. Smoke had to descend below the stove before rising in the flue, situated behind the stove.

Franklin was a great inventor, but his Pennsylvania Fireplace had a serious flaw: the problem was the smoke. According to Franklin’s design, smoke had to descend by means of a passageway cut through the floor before ascending in the flue. But smoke could not descend unless the floor was comparatively warm, which would provide the necessary draft. A cool floor meant that the smoke backed up into the stove and hence into the room.

Source: I. Bernard Cohen, Benjamin Franklin’s Science (Cambridge, Mass.: Harvard University Press, 1990).

Lightning Rod. Having shown that an object in a storm attracted an electric charge from clouds, Franklin advocated the use of iron rods to protect buildings and even ships from lightning strikes. Indeed, in September 1752 Franklin installed a lightning rod on his own house in Philadelphia. Franklin believed in grounded lightning rods to conduct the electricity from clouds through the rod to the ground. Franklin’s lightning rod rose nine feet above his chimney. The thin metal rod extended from the chimney through the staircase to his study, where it split into two rods, each with a bell at the end. Between the two bells hung a metal ball attached to a silk thread. When the lightning rod conducted an electric charge from a storm, the charge forced the ball to ring the bells. “One night,” Franklin wrote to a friend, “I was... awaked by loud cracks on the staircase.” He found not ringing bells but an intense electric charge of white light going from bell to bell, illuminating the entire staircase. Benjamin Franklin was the first in world history to use a

grounded lightning rod to protect a public or private building.