(Status of our work on Oscillatory Discharge and Radio Transmission as of summer of 2011.)
Who was Felix Savary?
Savary worked with Ampere when electromagnetism was just beginning. We focus on Savary’s study published in 1827.
What did he do?
Savary inferred that a charged Leyden jar would discharge in a damped oscillatory manner. This inference was based on observations of magnetization of short thin steel needles. Here needles were placed at varying distances near (and perpendicular) to a 2-meter-long wire loop used to discharge the Leyden jar. Magnetization of the needles would reverse direction (up to three times) as a function of distance from the wire in the loop. (Needles were placed close to the wire starting from a fraction of a mm to about 1 cm away.) (The remanent magnetization in these measurements results from magnetic hysteresis – more about this later.)
Why do we care?
Joseph Henry was stimulated by Savary’s observation. Henry repeated Savary’s work starting in 1835 and then extended it to magnetizing steel needles in a secondary circuit. Henry published his findings in 1842. Henry placed a steel needle in a spiral in a secondary circuit and removed the secondary circuit to a distance of 30 feet. The magnetizing of a steel needle at this distance is evidence of high frequency transmission and detection. We are working to determine the general range of frequencies of oscillation in Henry’s study, but it appears to be near 6 MHz. At 6 MHz the wavelength of the radiation is 50 meters (150 feet), so Henry’s observation of magnetization at a distance of 30 feet corresponds to the near field. This remote magnetization is evidence of high-frequency induction rather than radio transmission as is sometimes suggested. More to follow …
Some quantatative facts about Leyden jar discharge through a wire loop …
A Leyden jar with 200 pF capacitance, charged to a potential of 10KV, has a stored energy of 0.01 J. A circular wire loop of 2 mm dia, wire in a 1 meter dia, loop has an inductance of about 4 uH. So if the Leyden jar is discharged through this loop, the peak current would be about 70 A. The frequency of oscillation for this LC discharge circuit would be about 6 MHz. We still have to establish more precisely the values of C and L used by Savary and Henry, but these values are good starting points and they are match with our laboratory test conditions. Also, we note that the duration of the spark in the discharge (observed using a fast photo-diode) is about 1 usec. We observed a slow and a fast oscillation in the discharge circuit. The fast component seems to start after the spark has extinguished. While the spark is active, the detected signal is noisy – after the spark has extinguished the detected signal is much less noisy. The fast oscillation frequency is modified by changing the size of the spark gap.
Background Information
Felix Savary (1797- 1841) was a student and professor at the world-renowned French institution École Polytechnique. He is well known for his work in astronomy and contributions in electromagnetism. In recognition for his achievements in these fields, he was elected to the Académie des Sciences in 1832, joining the ranks of Joseph Fourier and Fracois Arago, who were also a members at the time.
A highly respected mathematician and physicist, Savary also worked with scientists such as André-Marie Ampère, one of the main discoverers of electromagnetism and who the SI unit of current is named after. From his joint research with Ampère, Savary went on to write Mémoire sur l’application du calcul aux phenomènes élecro-dynamique (1823).
Four years later, Savary’s most famous work, Mémoire sur l’Aimantation, was published in the French journal Annales de Chimie et de Physique. It was Savary who was the first to describe in this paper his hypothesis of the oscillatory nature of the discharge of a Leyden jar connected to an inductor. In his Mémoire, he documented the experiments which helped lead to his conclusion of the oscillatory discharge of the Leyden jar, which Joseph Henry expanded upon fifteen years later in America while working on his experiments in induction.
Felix Savary and Joseph Henry’s groundbreaking research in the oscillatory nature of the Leyden jar discharge laid the foundation to electrical resonance, which led to the developments and advancements in wireless telegraphy and the radio.
Click here for a more detailed explanation of Savary’s and Henry’s experiments.