The Hall effect is the phenomenon of the occurrence of a potential difference at the edges of a metal plate under the action of a magnetic field when an electric current is passed through it. Today it is used in keyboards, washing machines, cars. An interesting article about the Hall sensors.
The History of the Discovery of the
Effect On the discovery by Edwin Hall of such a specific effect, little is known. For some reason, such a significant event is not discussed in the literature. The section on Hall sensors mentions that Edwin made key observations during the doctoral degree at Johns Hopkins University in Baltimore. The event occurred in 1879.This is all that will be found in the literature regarding the origins of the great discovery.
Mentioned source, not so discussed. This is a note dated November 19th, in the autumn American Journal of Mathematics of 1879( Vol. 2, No. 3).Edwin speaks on pages 287-292 of the edition:
“Over the past year I have been studying a lot of Maxwell Electricity and magnetism, lectures by Professor Rowland. Separate lines hit the spotlight!“It is necessary to scrupulously note the fact that the force acting on a conductor with current, located across the magnetic field lines, is applied directly to the material. And if you apply voltage to a disk or a liquid, the material will begin to move obediently to the influence fully, and the nature of the movement may be consistent with the shape of the electric current, or be in dissonance with it. A constant magnetic force acts on the flow of charged particles. If the current were able to choose the path through the thickness of the material, then after a time it would return to the previous trajectory. The EMF of the source becomes the only real driving force. ”
The young scientist came to mind that the lines directly conflict with some already known phenomena. For the simple reason that the force acting on a wire with a current depends on the rate of flow of the charges. In contrast, the shape and configuration of the material acquire a small value. In turn, the interactions between charges are explained by their magnitude and sign, which has been known since the days of Charles Coulomb.
After Maxwell’s writings, Edwin’s note on Unipolar Induction( Annales de Chemie et de Physique, January 1879) comes across to Edwin Hall’s eyes. The text proved the fact that the magnet acts on a fixed conductor with a current of similar force, as if it were freely suspended. Hall forwarded the question to Professor Rowland and received in response a message about the employment of a scholar husband at the moment. Edwin had at his disposal a thought worthy of a riddle. Together with Professor Hall, he developed a methodology for the experiment:
If the current does not maintain a constant path of movement along the wire under the action of a magnetic field, the density of charges to one side will become higher. Which naturally increases the resistance of the conductor. Therefore, it remains to use Ohm’s law to test the hypothesis.
A flat wire helix( about half millimeter in diameter) of nickel silver( resembling a Tesla coil) with a total resistance of 2 ohms, sandwiched between two thick rubber pads, was chosen to implement the experiment. Sheet decided to place between the two poles of a magnet of a vast area. So that the lines of the field strength at each point are perpendicular to the direction of current flow. The electromagnet was powered by 20 Bunsen elements connected in 4 successive chains of 5 branches. The resulting intensity exceeded tens of thousands of times the horizontal component of the Earth’s magnetic field.
A measuring Whitston bridge was used as a sensor, the diagonal of which included a galvanometer of Lord Kelvin's design. The technical solution according to preliminary data recorded the change in the resistance of the helix in a millionth of the total value. From October 7 to 11, Edwin Hall did 13 experiments, each consisting of 40 measurements:
- Resistance Measurement with the magnet on.
- Similarly with the magnet turned off.
- P. 1 with a change in the polarity of the lines of the magnetic field.
- Repeats paragraph 2.
Measurements have shown that the magnetic field can reduce and increase the resistance. The maximum increase was fifteen hundredths, the average value on the basis of the experiments turned out to be much smaller( five ppm).It became clear that the actions taken were not enough to make certain statements. It is obvious that the current is hardly recognized as incompressible substance, as it was believed before. It was necessary to understand why the results of the first experiments are so different in meaning and direction of change in resistance.
The first Hall sensor
The first Hall sensor was designed by Professor Rowland. In the same form in which the device is used today. Seeing that Edwin's experiments( and his own) do not lead to the result, the lecturer suggested an old model of the experiment done over the years( the design of the Hall sensor is described):
- A conductive disk( or a plate of another shape) is switched on in the electric circuit.
- With the help of a galvanometer, two equipotential points are located on the sides of the figure.
- The electromagnet is turned on, the field strength lines of which lie in a plane perpendicular to the disk.
- Records changes in the readings of the galvanometer.
. It was supposed to detect signs of change when current flow conditions change. The experiment used the Hall sensor in the current performance, but the experience failed. It is believed that too much thickness of the disk is to blame. The professor brought this to Edwin's attention and expressed the opinion that the situation is repairable if we use a thin gold sheet mounted on a glass base( to prevent the metal from deforming the field).The experience of 28 October, which was completely successful, was able to fix a stable deflection of the galvanometer needle under the action of a magnetic field on a plate with a current.
And although the movement turned out to be permanent, it quickly disappeared, it was impossible to attribute this to magnetic induction( from Faraday's experiments).Quickly excluded the error introduced by the field of electric solenoids. On the horizon is clearly looming discovery. It is remarkable that the effect was inverted as the polarity of the magnet changed. To establish the quantitative dependences, the device was slightly improved:
- Strong contact of the power source was provided on each side with brass plates, well polished and carefully soldered to gold( 9x2 cm).
- A pure metal remained in the center: an area of 5.5 cm in length and across the entire width. Here the lines of magnetic field passed through gold.
- The Thomson high-resistance galvanometer contacts approached the edges equidistant from the brass plates.
During the experiment, the magnetic field of the solenoids, the currents through the plate and the galvanometer were measured. The result was recorded in the form of a table presented in the figure, showing that Edwin Hall managed to get the first patterns. It happened on November 12, 1879.Despite the fact that the expression on the right has values that differ by 8%, it is obvious that the order of the numbers is the same. And we will write off the deviations on the errors of experimenters and equipment.
Exact values are not always important. Today, Hall sensors are actively used as indicators of the absence or presence of a magnetic field. For example, in keyboards or engines of washing machines.
Applying the Hall Effect in Practice
Already said( see Hall sensors) that the first industrial applications of the Hall effect found their way into life in the second half of the 20th century. Today, just over half of the segment share is in the automotive industry. More precisely - advanced technologies in other areas come from there. For example, ASIC and ASSP modules. The leading role for the tenth year of the 21st century belongs to Asahi Kasei Microsystems( AKM), which supplies compasses for mobile devices based on the Hall effect. Among the industrial giants we note Micronas, Infineon, Allegro, Melexis. Among the magnetic field sensors based on the Hall effect occupy an honorary share of 87%.
Often the sensor is included in the chip. The historical ancestor is the CMOS series. On its basis, sensors integrated into the crystal were released to measure the angle of the throttle, steering, distribution and crankshaft rotation speeds. The technology is of great importance in the operation of valve engines, where the windings need to be switched in a certain way according to the angular position of the rotor. The measurement of the magnitude of the field involved the latest 3D-sensors that determine the angular and linear position of the system of magnets. Previously, it was simply the fact of the presence or absence of an object in sight. This is necessary for successful competition with magnetoresistive technology.
Today programmable constructions are considered to be the latest fashion, where different functions are entered by means of code. Sensors can be used in various ways. For example, according to the mutual position of the sensitive area and the magnet, there are modes:
- Frontal. In this case, the magnet is directly opposite the sensor, moving away from it or approaching in a straight line. The field depends quadratically on the distance and the law of the output signal from the distance resembles a hyperbole. This mode is called unipolar, tension can not change direction.
- Slip. In this case, there is a gap between the sensitive pad and the magnet. This coordinate remains unchanged. A magnet can slide parallel to the sensor on the same axis. In this case, the field does not change, and the dependence of the output signal on the coordinate is close to the Gaussian distribution. The direction of tension does not change, therefore the mode is also called unipolar.
- Bipolar Glide. Sometimes it is required to find out in which direction the magnet has deflected. And not only determine the distance. In this case, the magnet is used horseshoe. Accordingly, the poles produce responses of different polarities. What gave the name of the regime.
These modes are periodically used in combination. For example, when you need to accurately position the magnet relative to the sensors( using actuators), the sensitivity of the equipment increases with a steep characteristic of the dependence of the output signal on the coordinates. Three-band magnets with alternating poles are used. The extreme descents of the graph are gentle, and the central peak is pronounced. What is achieved accurate positioning of the system.
To strengthen the lines of tension, giving a clearly defined direction, pole tips are used. These are pieces of metal from soft ferromagnetic alloys. As the magnet approaches, the lines begin to strive toward the site, forming a gap where they remain straight. If you place the Hall sensor there, the sensitivity of the system increases significantly. For the same purpose, bias magnets are used, which remain in place and do not cause independent actuation. As the moving part approaches, the density of the magnetic field increases sharply. This simplifies triggering and reduces sensor sensitivity requirements.
Add that the structure of the output signal sensors are analog and digital. In the latter case, the system easily mates with automation, and the measured signal no longer loses accuracy, being transferred for processing.