3.1 The Dynatron#

One of Hull’s early significant inventions was the Dynatron, developed around 1918.

Dynatron: A type of vacuum tube utilizing secondary emission to produce a region of negative differential resistance.

• Mechanism: Standard vacuum tubes (like triodes or tetrodes) typically exhibit positive resistance – as voltage increases, current increases. The Dynatron exploited secondary emission. When electrons from the filament (cathode) strike a plate (anode), they can knock off additional electrons from the anode’s surface. In the Dynatron design, a grid is placed between the cathode and the anode, and the anode is held at a lower potential than this grid. Electrons accelerated towards the high-potential grid pass through its mesh and strike the anode. If the anode voltage is sufficiently low relative to the grid voltage, the secondary electrons emitted from the anode are attracted away from the anode and towards the higher-potential grid. This means that as the anode voltage increases slightly in a certain range, the net current flowing into the anode decreases (because more secondary electrons are leaving than incoming primary electrons are increasing the current). This phenomenon creates a region of negative differential resistance.

• Negative Differential Resistance:

  Negative Differential Resistance (NDR): A property of certain electronic components where, over a specific range of applied voltage, an increase in voltage causes a decrease in electric current. This is the opposite of typical materials (like resistors) which exhibit positive resistance (Ohm’s Law).

  NDR is a crucial concept in electronics. A component exhibiting NDR can supply power to an AC signal, making it suitable for building oscillators.

• Applications: The Dynatron was one of the first devices to exploit NDR for practical applications. It was used to build oscillators (circuits that generate repeating electronic signals, like sine waves) and regenerative amplifiers (amplifiers that feed a portion of the output back to the input to increase gain or selectivity). Its ability to generate oscillations from a simple circuit topology made it valuable before more complex feedback amplifier designs became common.

• Significance: The Dynatron demonstrated a novel way to achieve oscillation using a specific tube characteristic rather than relying solely on external feedback networks. While later tubes and semiconductor devices became more popular for NDR applications, the Dynatron was a pioneering example.

3.2 The Magnetron#

Albert Hull invented an early form of the Magnetron in 1921.

Magnetron: An electron tube in which the flow of electrons is controlled by both an electric field and a magnetic field, typically used for generating microwaves.

• Mechanism: Hull’s initial design was often a “split-anode” magnetron. It consisted of a cylindrical anode divided into two or more segments, surrounding a central cathode filament. A uniform magnetic field was applied parallel to the filament axis. Electrons emitted from the cathode are accelerated radially outwards by the electric field towards the anode segments. However, the magnetic field exerts a force perpendicular to both the electron’s velocity and the magnetic field vector (the Lorentz force). This causes electrons to travel in curved paths. By carefully adjusting the strength of the magnetic field, the electron trajectories could be controlled. In certain conditions, electrons might loop back towards the cathode or spiral towards the anode segments in complex paths. Hull’s split-anode design could be used to generate oscillations by causing electrons to oscillate between the anode segments.

• Electric and Magnetic Field Interaction: This device is a classic example of how crossed electric and magnetic fields influence charged particle motion, a fundamental concept in physics and electrical engineering (relevant to topics like Hall effect sensors, particle accelerators, and plasma physics).

• Applications: Hull’s magnetron was capable of generating relatively high-frequency oscillations (early radio frequencies). While his design was not the multi-cavity resonant magnetron that became famous for radar in World War II and later used in microwave ovens, his fundamental work on controlling electron flow using magnetic fields in a diode-like structure was foundational to the development of later, more powerful microwave-generating magnetrons. His split-anode version was used as an oscillator in some early radio systems.

• Significance: The magnetron was one of the first devices capable of generating significant power at radio frequencies extending into what would later be called the “microwave” range, opening up new possibilities for communication and sensing.

3.3 The Thyratron#

Developed by Hull around 1928, the Thyratron was a significant step forward in controlled power switching.

Thyratron: A gas-filled electron tube with three or more electrodes (typically cathode, grid, and anode), used primarily as a high-current electronic switch or rectifier.

Gas-filled tube: An electron tube containing a small amount of inert gas (like argon, neon, or mercury vapor) which ionizes when sufficient voltage is applied, allowing much higher current flow than a vacuum tube for the same size.

• Mechanism: Like a vacuum triode, the Thyratron has a cathode (electron source), an anode (plate), and a grid (control electrode). However, it contains a low-pressure inert gas. In a vacuum triode, the grid controls the continuous flow of current. In a Thyratron, the grid acts like a trigger. With no positive voltage on the grid (or a negative bias), the tube is non-conductive – the gas atoms are neutral. When a sufficiently positive voltage is applied to the grid (relative to the cathode), it initiates a stream of electrons towards the anode. These electrons collide with gas atoms, ionizing them (stripping off electrons). This creates a plasma of positive ions and electrons. The positive ions drift towards the cathode and grid, neutralizing the negative space charge that limits current in vacuum tubes and effectively shielding the grid. Once the gas is fully ionized (“struck” or “fired”), the impedance of the tube drops dramatically, and it conducts a large current between cathode and anode. Crucially, the grid loses control once the tube is firing. The only way to stop conduction is to reduce the anode voltage below a minimum holding level, causing the gas to de-ionize.

• Comparison to Vacuum Tubes: Unlike a vacuum triode which can amplify signals linearly or switch on/off under continuous grid control, the Thyratron is a non-linear, latching switch. The grid only determines when conduction starts, not how much current flows (once on) or when it stops.

• Applications: Thyratrons were widely used for controlling large amounts of power in applications like:

  • Controlled Rectification: Used in AC circuits to control the average DC voltage delivered to a load by varying the phase angle at which the Thyratron is triggered during each AC half-cycle. This was an early form of phase control.
  • Motor Speed Control: By controlling the firing angle in AC or DC circuits driving motors.
  • Relay and Contactor Control: Switching high-current loads with a low-power control signal.
  • Ignition and Triggering Circuits: Firing other devices like ignitrons or triggering pulsed systems.
  • Inverters and Cycloconverters: Early forms of power electronic circuits converting DC to AC or AC of one frequency to AC of another.

• Significance: The Thyratron was one of the first practical high-power electronic switching devices. It enabled precise electronic control of power levels in industrial applications previously only possible with less efficient or slower mechanical/electromagnetic methods. While largely superseded by semiconductor devices like Silicon Controlled Rectifiers (SCRs) and Triacs, Thyratrons pioneered many power control techniques still used today.