Nikola Tesla: Colorado Springs Notes, 1899-1900 Page 109

July 27

In his first condenser discharge oscillation transformer for generating high frequencies in 1891^{(4, 15)} Tesla used a simple air gap for regulating the charging and discharging of the condenser. However, a year later he had already described several improvements on simple spark gaps using a magnetic field or an air current for rapid extinction of the arc thereby reducing the period of the charge-discharge cycle. He also described the advantages of a splitted arc across several smaller air gaps: with the same total gap length the breakdown voltage is higher, so that smaller gaps can be used and the losses are less*. A fourth form of improvement which he invented was the use of various rotary interrupters⁽⁵⁾.

In the period from 1893 through 1898 Tesla patented several types of interrupter, or “electric circuit controllers”. It is interesting that all these patents refer to various types of rotary interrupter, with or without an air gap. Some rotary interrupters were protected within patents for high-frequency generators, including the following:

the combination with discharge points immersed in oil. The turbine whose blades make and break the condenser circuit is driven by oil under pressure⁽⁵⁰⁾

mechanical make-break controllers for DC⁽⁵¹⁾

synchronous controllers with and without regulation of the interrupt timing, for use with AC sources⁽⁵²⁾

commutators for alternate switching between two condensers in the primary circuit of a Tesla oscillator⁽⁵³⁾

In 1897 and 1898 Tesla was granted a number of patents for “electric circuit controllers” The principle requirement was that they should make and break a circuit at the highest possible rate, i.e. that they should perform a large number of operations per unit time. In eight patents⁽²⁷⁾ Tesla gives designs for rotary interrupters with conducting or conducting and insulating fluids, usually mercury and oil, respectively. In some designs the interruption takes place in an inert gas under pressure. He gives ingenious designs for using a mercury jet playing on a toothed metal rotor, and for producing two mercury jets (fluid contact).

The rotary interrupter with two auxiliary air gaps shown in the figure was a new idea. One of the reasons Tesla added these air gaps was probably the high voltages with which he was working, since they allowed him to regulate the excitation. That this could be done may be seen from the statement that by adjusting these gaps the period of charging from the secondary of the mains transformer could be shortened. At the end of the entry he records that the best results were obtained with two rotary interrupters (with toothed disks) rotating in opposite directions. He does not explain how he chose the tooth ratio so that the number of interruptions was equal to the product of the number of teeth.

* The total resistance series air gaps is less than the resistance of a single air gap with the same breakdown voltage.

July 27

In his first arcing-type oscillator for high frequency current production in 1891^(4,15) Tesla used a simple "air gap" for the regulation of the oscillator capacitor charging and discharging. Already in the next year he describes several methods for improved operation of simple arcing devices by application of a magnetic field, or an air stream or quick spark interruption and the increase of capacitor charge and discharge frequency. Also he described the advantages of a "divided spark" in multiple small air gaps, when due to increased flashover voltage of a divided arcing device* in comparison with simple arcing device (of the same total air gap length) small gaps can be applied and the losses are smaller.

Fourth method for the achievement of capacitor charging and discharging improved system consisted in the application of various rotating breakers⁽⁵⁾.

In patents over the period 1893-1898 Tesla protected several types of breakers or "electrical circuit regulators". It is interesting that all these patents pertain to various types of rotating breakers (with air gap and without). Some of the rotating breakers are protected in an assembly of apparatus for high frequency currents production. Among them are:

• Rotating arcing device (breaker without contacts) in oil made in the shape of a turbine, over which blades the capacitor circuit is made and interrupted, and it rotated by means of oil under pressure⁽⁵⁰⁾.
• Mechanical regulators of electrical circuit of the "on-off" type for operation with direct current⁽⁵¹⁾.
• Synchronous regulators of electrical circuits with regulation of the instant of interruption for operation with alternating current sources or without it⁽⁵²⁾.
• The commutators for alternating "on-off" switching, or connection of two capacitors in Tesla's oscillator primary circuit⁽⁵³⁾.

In 1897 and 1898 Tesla obtained a number of patents in the field of "electrical circuit regulators. The basic requirement is that these regulators establish and interrupt electrical circuits at the highest possible speed. Further it is required to achieve a great number of operations per time unit. In eight patents⁽²⁷⁾ Tesla described the design of rotating breakers with conductive or nonconductive fluids. conductive fluid is usually mercury and nonconductive oil. At some regulators the interruption is performed in inert gas under pressure. Extraordinary are the solutions for the achievements of microjets which create a contact with two rotors made of metal for the achievement of two microjets (fluid contact).

Rotating arcing device with two auxiliary gaps as on the Figure is one new solution. Probably additional gaps were applied by Tesla in order to regulate by means of them, the excitation during operation with high voltages. It can be seen from the text that this is possible to achieve when it is mentioned that by means of those gaps it is possible to shorten the supply interval from the network transformer secondary. At the end of his consideration Tesla says he achieved the best results with two rotating arcing devices, (tooth shaped) which rotate in opposite directions. He didn't explain how he chose the number of "teeth" on each rotating disc in order to get the number of interruptions equal to the product of the number of "teeth".

* The total resistance of air gaps connected in series at multiple gap arcing device is smaller than the resistance of usual arcing device of the same flashover voltage.

July 28

This entry provides one of the most detailed descriptions of the receiver with two rotating coherers and a condenser for accumulating the energy from weak signals. At point b the circuit C - P is periodically made and broken and the resulting AC pulses bias sensitive device A' in the secondary. Sensitive device A is still poorly conducting so the charging current of C via damping coil L is small. When an arriving electromagnetic wave reduces the resistance of A, C charges much faster and the voltage induced in secondary S also rises rapidly. The resistance of A' drops rapidly and current from battery B' activates relay R. Judging by Tesla's report, the receiver was very sensitive to distant electrical discharges.

July 28

This is one of the most detailed descriptions of a receiver with two rotating sensitive devices and capacitor for effective accumulation of weak signals. At location 'b' the circuit C - P is periodically switched on and interrupted and obtained alternating current impulses pre-excite the sensitive device A' in the secondary. In the non-excited state, other sensitive device A has high resistance* and therefore the charging current C via ballast coil L is low. When the resistance of sensitive device A reduces, due to action of electromagnetic waves, capacitor charging current C quickly increases and the induced voltage in secondary S is quickly increased as well.

Sensitive device A' resistance is quickly reduced, and current from battery B' acts on relay R which is activated.

Judging on the basis of what Tesla displays, the receiver is very sensitive and reacts to remote electrical discharges.