In physics a null is a point in a

field Field may refer to: Expanses of open ground * Field (agriculture), an area of land used for agricultural purposes * Airfield, an aerodrome that lacks the infrastructure of an airport * Battlefield * Lawn, an area of mowed grass * Meadow, a grass ...

where the field quantity is zero as the result of two or more opposing quantities completely cancelling each other. The field may be

scalar Scalar may refer to: *Scalar (mathematics), an element of a field, which is used to define a vector space, usually the field of real numbers *Scalar (physics), a physical quantity that can be described by a single element of a number field such a ...

vector Vector most often refers to: * Euclidean vector, a quantity with a magnitude and a direction * Disease vector, an agent that carries and transmits an infectious pathogen into another living organism Vector may also refer to: Mathematics a ...

tensor In mathematics, a tensor is an algebraic object that describes a multilinear relationship between sets of algebraic objects associated with a vector space. Tensors may map between different objects such as vectors, scalars, and even other ...

in nature. Common situations where nulls arise are in the polar patterns of

microphone A microphone, colloquially called a mic (), or mike, is a transducer that converts sound into an electrical signal. Microphones are used in many applications such as telephones, hearing aids, public address systems for concert halls and publi ...

s and antennae, and nulls caused by reflections of

waves United States Naval Reserve (Women's Reserve), better known as the WAVES (for Women Accepted for Volunteer Emergency Service), was the women's branch of the United States Naval Reserve during World War II. It was established on July 21, 1942, ...

Microphones

A common polar pattern for microphones is the

cardioid In geometry, a cardioid () is a plane curve traced by a point on the perimeter of a circle that is rolling around a fixed circle of the same radius. It can also be defined as an epicycloid having a single cusp. It is also a type of sinusoidal ...

. This has a single direction in which the microphone does not respond to impinging sound waves. Highly directional (shotgun) microphones have more complex polar patterns. These microphones have a large, narrow lobe in the main direction of sound reception but also a smaller lobe in the opposite direction and usually also several other smaller lobes. This pattern is achieved by wave cancellation inside the body of the microphone. Between each of these lobes is a null direction where no sound at all is detected.

Antennae

A common, and basic, radio antenna is the

dipole In physics, a dipole () is an electromagnetic phenomenon which occurs in two ways: * An electric dipole moment, electric dipole deals with the separation of the positive and negative electric charges found in any electromagnetic system. A simple ...

. This has a figure-of-eight polar pattern with two nulls on opposite sides. Highly directional antennae, such as the Yagi have polar patterns very similar to highly directional microphones and for similar reasons. That is, they have multiple small lobes off the main direction with nulls between them.

Standing waves

Onde stationnaire pression tuyau ferme trois modes

Standing wave In physics, a standing wave, also known as a stationary wave, is a wave that oscillates in time but whose peak amplitude profile does not move in space. The peak amplitude of the wave oscillations at any point in space is constant with respect t ...

s can be caused by a wave being reflected back through the transmission medium in which it arrived. If the incident and reflected waves are transmitted without loss then there will be points along the transmission path where the incident and reflected waves exactly cancel each other due to them being in

antiphase In physics and mathematics, the phase (symbol φ or ϕ) of a wave or other periodic function F of some real variable t (such as time) is an angle-like quantity representing the fraction of the cycle covered up to t. It is expressed in such a s ...

. Standing waves are found in wind musical instruments. Wind instruments consist of a tube which acts as an acoustic transmission line in which standing waves are set up. Open ended tubes must have zero air pressure change at the end of the tube so this point is a pressure null. Closed tubes must have zero air velocity at the end of the tube so this point is a velocity null. There may be further nulls along the tube depending on the vibration mode that has been set up by the instrument player. Higher modes have more nulls. Standing waves also occur on electrical

transmission line In electrical engineering, a transmission line is a specialized cable or other structure designed to conduct electromagnetic waves in a contained manner. The term applies when the conductors are long enough that the wave nature of the transmis ...

s. A line with a non-

resistive The electrical resistance of an object is a measure of its opposition to the flow of electric current. Its reciprocal quantity is , measuring the ease with which an electric current passes. Electrical resistance shares some conceptual paral ...

termination will cause a

reflection Reflection or reflexion may refer to: Science and technology * Reflection (physics), a common wave phenomenon ** Specular reflection, mirror-like reflection of waves from a surface *** Mirror image, a reflection in a mirror or in water ** Diffuse r ...

of the signal from the termination with the same amplitude as the incident wave. These waves will cancel periodically along the line causing nulls every half

wavelength In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats. In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...

. The distance of the first null from the termination depends on the nature of the terminating impedance. Nulls on transmission lines are very sharp, in contrast to the peaks which are broad and flat. This makes the nulls easier to measure. An instrument in electronics for measuring the positions of nulls on a line is the

slotted line Slotted lines are used for microwave measurements and consist of a movable probe inserted into a slot in a transmission line. They are used in conjunction with a microwave power source and usually, in keeping with their low-cost application, a ...

. This instrument can also be used to measure

VSWR In Radio-frequency engineering, radio engineering and telecommunications, standing wave ratio (SWR) is a measure of impedance matching of Electrical load, loads to the characteristic impedance of a transmission line or waveguide. Impedance misma ...

. With the measurement of VSWR and the null positions the value of the terminating impedance's magnitude and phase can be calculated.

Measuring bridges

Nulls are used in electrical science to make many measurements. A frequently encountered technique is to adjust the voltage in one branch of a circuit until it nulls out a voltage in another branch. Commonly, a bridge circuit is used for this purpose. There are many varieties of measurement bridge, the most well known of these being the

Wheatstone bridge A Wheatstone bridge is an electrical circuit used to measure an unknown electrical resistance by balancing two legs of a bridge circuit, one leg of which includes the unknown component. The primary benefit of the circuit is its ability to prov ...

used to accurately measure the resistance of an unknown component against a calibrated variable resistance by comparing the voltages across each. The central branch of the bridge has the voltage under test on one side and the calibrated voltage on the other: when they are equal the voltage across the central branch is nulled out. The measurement is carried out by placing an instrument to detect voltage or current in the central branch. An advantage of the bridge method is that this instrument does not need to be calibrated since it is only required detect nulls, not to actually measure a voltage. The accuracy of the measurement will, however, depend on the sensitivity of the instrument since this will affect the resolution of the bridge. Electromagnetism