Bolometer

A bolometer consists of an absorptive element, such as a thin layer of metal, connected to a thermal reservoir (a body of constant temperature) through a thermal link. The result is that any radiation impinging on the absorptive element raises its temperature above that of the reservoir — the greater the absorbed power, the higher the temperature. The intrinsic thermal time constant, which sets the speed of the detector, is equal to the ratio of the heat capacity of the absorptive element to the thermal conductance between the absorptive element and the reservoir.  The temperature change can be measured directly with an attached resistive thermometer , or the resistance of the absorptive element itself can be used as a thermometer. Metal bolometers usually work without cooling. They are produced from thin foils or metal films. Today, most bolometers use semiconductor or superconductor absorptive elements rather than metals. These devices can be operated at cryogenic temperatures, enabling significantly greater sensitivity.

Bolometers are directly sensitive to the energy left inside the absorber. For this reason they can be used not only for ionizing particles and photons, but also for non-ionizing particles, any sort of radiation, and even to search for unknown forms of mass or energy (like dark matter ); this lack of discrimination can also be a shortcoming. The most sensitive bolometers are very slow to reset (i.e., return to thermal equilibrium with the environment). On the other hand, compared to more conventional particle detectors, they are extremely efficient in energy resolution and in sensitivity. They are also known as thermal detectors.

Ballistic Galvanometer

Ballistic galvanometer

A ballistic galvanometer is a type of sensitive galvanometer, commonly a mirror galvanometer . Unlike a current-measuring galvanometer, the moving part has a large moment of inertia , thus giving it a long oscillation period. It is really an integrator measuring the quantity of charge discharged through it. It can be either of the moving coil or moving magnet type.

Grassot Fluxmeter

An interesting form of ballistic galvanometer is the Grassot fluxmeter. In order to operate correctly, the discharge time through the regular ballistic galvanometer must be shorter than the period of oscillation. For some applications, especially those involving inductors, this condition cannot be met. The Grassot fluxmeter solves this. Its construction is similar to that of a ballistic galvanometer, but its coil is suspended without any restoring forces in the suspension thread or in the current leads. The core (bobbin) of the coil is of a non-conductive material. When an electric charge is connected to the instrument, the coil starts moving in the magnetic field of the galvanometer's magnet, generating an opposing e.m.f. and coming to a stop regardless of the time of the current flow. The change in the coil position is proportional only to the quantity of charge. The coil is returned to the zero position by the reversing of the current or manually.

Flux Meter

Principle of Fluxmeters

As magnetic flux cuts through the search coil, it induces a voltage in the search coil. As per Faradays law of induction, this voltage is the differential of the magnetic flux that passed through the search coil. By feeding this voltage into an integrating fluxmeter, the integration process removes the differential (of the search coil) resulting in the fluxmeter displaying the total magnetic flux. Fluxmeters require a dynamic component for the measurement, as it is necessary for magnetic flux to cut the search coil to produce a voltage.

By calibrating the fluxmeter with the area and number turns of the search coil it is possible to display values of flux density on a fluxmeter (Tesla, gauss) as well as magnetic flux (Webers, Vs or Maxwell turns)

Order Of Instruments

Zero Order Instruments

A zero order linear instrument has an output which is proportional to the input at all times in accordance with the equation

y(t) = Kx(t)

where K is a constant called the static gain of the instrument. The static gain is a measure of the sensitivity of the instrument.

An example of a zero order linear instrument is a wire strain gauge in which the change in the electrical resistance of the wire is proportional to the strain in the wire.

All instruments behave as zero order instruments when they give a static output in response to a static input.

First Order Instruments

A first order linear instrument has an output which is given by a non-homogeneous first order linear differential equation

tau .dy(t)/dt + y(t) = K.x(t)

where tau is a constant, called the time constant of the instrument.

In these instruments there is a time delay in their response to changes of input. The time constant tau is a measure of the time delay.

Thermometers for measuring temperature are first-order instruments. The time constant of a measurement of temperature is determined by the thermal capacity of the thermometer and the thermal contact between the thermometer and the body whose temperature is being measured.

A cup anemometer for measuring wind speed is also a first order instrument. The time constant depends on the anemometer's moment of inertia.

Second Order Instruments

A second order linear instrument has an output which is given by a non-homogeneous second order linear differential equation

d 2y(t)/dt 2 + 2. rho .omega.dy(t)/dt +omega 2.y(t) = K. omega2.x(t)

where rho is a constant, called the damping factor of the instrument, and omega is a constant called the natural frequency of the instrument.

Under a static input a second order linear instrument tends to oscillate about its position of equilibrium. The natural frequency of the instrument is the frequency of these oscillations.

Friction in the instrument opposes these oscillations with a strength proportional to the rate of change of the output. The damping factor is a measure of this opposition to the oscillations.

An example of a second order linear instrument is a galvanometer which measures an electrical current by the torque on a coil carrying the current in a magnetic field. The rotation of the coil is opposed by a spring. The strength of the spring and the moment of inertia of the coil determine the natural frequency of the instrument. The damping of the oscillations is by mechanical friction and electrical eddy currents.

Excitation

An electric generator or electric motor consists of a rotor spinning in a magnetic field. The magnetic field may be produced by permanent magnets or by field coils. In the case of a machine with field coils, a current must flow in the coils to generate the field, otherwise no power is transferred to or from the rotor. The process of generating a magnetic field by means of an electric current is called excitation .

Except for permanent magnet generators, a generator produces output voltage proportional to the magnetic field, which is proportional to the excitation current; if there is no excitation current there is zero voltage. A small amount of (electric) power may control a large amount of power. This principle is very useful for voltage control: if the system voltage is low, excitation can be increased; if the system voltage is high, excitation can be decreased. A synchronous condenser operates on the same principle, but there is no "prime mover" power input; however, the "flywheel effect" means that it can send or receive power over short periods of time.

Self excitation

Modern generators with field coils are self-excited , where some of the power output from the rotor is used to power the field coils. The rotor iron retains a magnetism when the generator is turned off. The generator is started with no load connected; the initial weak field creates a weak voltage in the stator coils, which in turn increases the field current, until the machine "builds up" to full voltage.

Starting

Self-excited generators must be started without any external load attached. An external load will continuously drain off the buildup voltage and prevent the generator from reaching its proper operating voltage.

Field flashing

If the machine does not have enough residual magnetism to build up to full voltage, usually a provision is made to inject current into the rotor from another source. This may be a battery , a house unit providing direct current , or rectified current from a source of alternating current power. Since this initial current is required for a very short time, it is called "field flashing". Even small portable generator sets may occasionally need field flashing to restart.

The critical field resistance is the maximum field circuit resistance for a given speed with which the shunt generator would excite. The shunt generator will build up voltage only if field circuit resistance is less than critical field resistance. It is a tangent to the open circuit characteristics of the generator at a given speed.

Slip Ring

A slip ring is an electromechanical device that allows the transmission of power and electrical signals from a stationary to a rotating structure. A slip ring can be used in any electromechanical system that requires rotation while transmitting power or signals. It can improve mechanical performance, simplify system operation and eliminate damage-prone wires dangling from movable joints.

Also called rotary electrical interfaces, rotating electrical connectors , collectors , swivels , or electrical rotary joints , these rings are commonly found in slip ring motors, electrical generators for alternating current (AC) systems and alternators and in packaging machinery, cable reels, and wind turbines . They can be used on any rotating object to transfer power, control circuits, or analog or digital signals including data such as those found on aerodrome beacons, rotating tanks , power shovels , radio telescopes or heliostats .

A slip ring is a method of making an electrical connection through a rotating assembly. Formally, it is an electric transmission device that allows energy flow between two electrical rotating parts, such as in a motor.

Thermal Anemometers

Thermal anemometers use a very fine wire (on the order of several micrometers) or element heated up to some temperature above the ambient. Air flowing past over has a cooling effect. As the electrical resistance of most metals is dependent upon the temperature of the metal (tungsten is a popular choice for hot wires), a relationship can be obtained between the resistance of the wire and the flow velocity.

Several ways of implementing this exist, and hot-wire devices can be further classified as CCA (Constant-Current Anemometer), CVA (Constant-Voltage Anemometer) and CTA (Constant-Temperature Anemometer). The voltage output from these anemometers is thus the result of some sort of circuit within the device trying to maintain the specific variable (current, voltage or temperature) constant. Additionally, PWM (Pulse Width Modulation) anemometers are also used, wherein the velocity is inferred by the time length of a repeating pulse of current that brings the wire up to a specified resistance and then stops until a threshold "floor" is reached, at which time the pulse is sent again.

Hot-wire anemometers, while extremely delicate, have extremely high frequency-response and fine spatial resolution compared to other measurement methods, and as such are almost universally employed for the detailed study of turbulent flows, or any flow in which rapid velocity fluctuations are of interest. Thermal anemometers are available with additional functions such as temperature measurement, data logging ability.

Residual Magnetism

Unlike the separately excited generator, there is no current in the field circuit when the armature is motionless. Since a small amount of residual magnetism is present in the field poles, a weak residual voltage is induced in the armature as soon as the armature is rotated. This residual voltage produces a weak current in the field circuit. If this current is in the proper direction, an increase in magnetic strength occurs with a corresponding increase in voltage output. The increased voltage output, in turn, increases the field current and the field flux which, again, increase the voltage output. As a result of this action, the output voltage builds up until the increasing field current saturates the field poles. Once the poles are saturated, the voltage remains at a constant level, unless the speed of the armature rotation is changed.

If the direction of armature rotation is reversed, the brush polarity also is reversed. The residual voltage now produces a field current which weakens the residual magnetism and the generator voltage fails to build up. Therefore, a self-excited machine develops its operating voltage for one direction of armature rotation only. The generator load switch may be closed when the desired voltage is reached.

GAUSSMETER

A gaussmeter is also called as a magnetometer. A magnetometer is a scientific instrument used to measure the strength and/or direction of the magnetic field in the vicinity of the instrument.

A direct current flowing in an inductor creates a strong magnetic field around a hydrogen-rich fluid, causing the protons to align themselves with that field. The current is then interrupted, and as protons are realigned with Earth's magnetic field they precess at a specific frequency. This produces a weak alternating magnetic field that is picked up by a (sometimes separate) inductor. The relationship between the frequency of the induced current and the strength of Earth's magnetic field is called the proton gyromagnetic ratio, and is equal to 0.042576 hertz per nanotesla (Hz/nT).

Inductive Pickup Coils

Inductive pickup coils measure the magnetization by detecting the current induced in a coil due to the changing magnetic moment of the sample. The sample’s magnetization can be changed by applying a small ac magnetic field (or a rapidly changing dc field), as occurs in capacitor-driven pulsed magnets. These measurements require differentiating between the magnetic field produced by the sample and that from the external applied field. Often a special arrangement of cancellation coils is used. For example, half of the pickup coil is wound in one direction, and the other half in the other direction, and the sample is placed in only one half. The external uniform magnetic field will be detected by both halves of the coil and since they are counterwound the external magnetic field produces no net signal.

In 1833, Carl Friedrich Gauss , head of the Geomagnetic Observatory in Göttingen, published a paper on measurement of the Earth's magnetic field.  It described a new instrument that consisted of a permanent bar magnet suspended horizontally from a gold fibre. The difference in the oscillations when the bar was magnetised and when it was demagnetised allowed Gauss to calculate an absolute value for the strength of the Earth's magnetic field.