Used simply to measure and record the temperature. Several types are available from the basic liquid in glass types through to hi-spec digital models with milli degree accuracy and computer interfacing.
The main types that we are concerned with are direct reading and indirect reading.
The direct reading
The direct reading type makes use of some form of sensor that is placed in contact with the medium being measured i.e. the air or a surface. There are several types of sensor that can be used including a probe that consists of two dissimilar metals - called a thermocouple. The probe is placed directly into an area where the temperature is to be measured - a room for example. Any change in temperature in the room causes a difference in the conductivity of the thermocouple and this can be read on a digital scale calibrated in degrees Celsius or Fahrenheit. Other types of sensor may use changes in resistance within the probe (a thermistor type) to measure the temperature.
Some models are able to record the maximum and minimum temperatures, other models even record the temperature many times every second and can either be read directly using a connected computer or stored within a solid state memory inside the unit for later downloading to a PC, where the recorded temperature over many hours can be displayed as a graph.
The indirect reading,
The indirect reading, more commonly called the non-contact type looks similar to a 'Star Wars' laser gun and in fact most models even shoot out a beam of laser light. The laser is simply there to act as a guide to what the thermometer is pointing at by conveniently placing a red laser spot or ring onto the surface being measured.
A special heat sensitive element similar to that used in PIR (Passive Infra Red) motion detectors 'looks' at the place where the gun is pointing and measures the temperature from the amount of reflected Infra Red energy being emitted. All objects emit heat in the form of IR energy which can be 'seen' in this way, although there may be some large variations in the amounts of IR that it emitted in some types of material. This is called the emissivity value and is expressed as a numeric value. Most non-contact thermometers are calibrated for an average emissivity of 0.95 which will give a fairly accurate temperature for most types of surface that one may wish to measure. Because of the emissivity value is not a constant then non-contact thermometers may have varying degrees of accuracy depending upon the surface they are being used with. Some of the current models of non-contact thermometer available allow the user to manually select the emissivity calibration of the device prior to use. Tables are available from the equipment makers that give examples of the emmisivity values of common materials.
Why measure temperature?
There is certainly a good deal of anecdotal evidence to link sudden changes in ambient temperature to reports of people experiencing unusual or possibly even paranormal events. Interestingly, there is even some (limited) objective data from calibrated thermometers that have recorded seemingly inexplicable large or sudden fluctuations of the temperature at some locations.
Many witnesses to alleged paranormal activity report that the ambient temperature changes suddenly, often dropping but in some cases rising, just before, during or just after an anomalous event took place. "The room went icy cold, then the ghost or spirit appeared" or "then the object moved" is for example often reported by witnesses to haunts, apparitions and at séances.
The reported temperature change may be a real event or it may be something that is only perceived by the witness. A witness may report feeling 'icy' cold and no temperature change can be objectively measured. This may be simply because the probe is in the wrong place or it may be that the wrong sort of thermometer is being used - we cannot for example measure the temperature of the air using an indirect reading thermometer as it will only give a reading for a solid or IR emitting surface.
Additionally, there are several physical mechanisms that can cause the temperature to change or be perceived to be changing and that are much harder to measure by conventional means. The 'Peltier effect' is one example; this uses the flow of electrons to effect cooling and is nowadays being increasingly used in cooling of computer components. Peltier cooling is difficult to measure with standard equipment. Another mechanism, uses the flow of charged air ions, creating an 'ion wind' that can be felt as a gentle cool breeze by a person but cannot be measured by normal thermometers - place your hand close to a domestic air ioniser to try this out for yourself.
There are also many physiological and psychological reasons that may cause a person to feel cold when in fact the temperature has not changed. Fear, excitement or drugs all may cause the blood supply to the skin to alter and this may cause the witness to feel colder or hotter. Simply the belief that a location is haunted and that the temperature is 'supposed to drop' when ghosts appear may be enough for some people to report feelings of extreme cold.
The language that witnesses may use can also be highly misleading too - they often choose extreme descriptions - "It went ICY cold". "It was like walking into a freezer" are commonly encountered phrases in paranormal investigations. The reality, when measured may be a small temperature change of just a degree or two, perhaps even no change at all. It is not unnatural for people when trying to deal with things that are beyond their normal range of comprehension to offer exaggerated accounts to try and convey the magnitude or fantastic nature of their experience.