144 



Pseudophone. The name given by Professor Silvanus Thompson 

 to an instrument illustrating the laws of the acoustic per- 

 ception of space by the illusions it produces. 



Pseudoscope. An instrument which exhibits objects in their 

 proper relief reversed. [26.] 



Psychrometer. A wet and dry bulb hydrometer (q.v.). [4, 5, 

 13, 15, 16, 17, 20, 21, 27.] 



Psychrometer, Assmann's. A form of wet and dry bulb hygro- 

 meter (q.v.), in which a spring-driven fan is used to draw air 

 over the bulbs of the thermometers. [6, 17, 21.] 



Pulmometer. See Spirometer. [21.] 



Pulsimeter. See Sphygmometer. [21.] 



Pupilometer. An instrument for measuring the size of the pupil 

 of the eye. [26.] 



Pycnometer. A specific gravity bottle for measuring and com- 

 paring the densities of liquids. [21.] 



Pyknometer. See Pycnometer. [21.] 



Pyrheliometer. An instrument for measuring the amount of 

 heat radiated from the sun. [21] 



Pyrheliometer, Abbe's Silver Disc. 



Pyrheliometer, Angstrom's. The essential part of this instrument 

 consists of two strips of thin metal, one of which is exposed 

 to the sun's rays and the other, kept in the shade, is heated 

 by an electric current and kept at the same temperature as 

 the exposed slip. 



Pyrograph. A curve indicating measurements of heat. [17.] 



Pyrometer. Instrument used for the measurement of temper- 

 atures higher than those for which a glass thermometer is 

 suitable. In its simplest form it depends on the expansion 

 of a solid metal rod or of a gas, but more commonly it 

 consists of an electrical device. The pyrometer in most 

 common use depends on the electrical current generated 

 by a junction of two different metals or alloys when it is 

 heated. This pattern will indicate temperatures up to 

 about 1,350 or 1,400 C. For temperatures higher than 

 that, Radiation Pyrometers are used. In this form, the 

 heat is focussed by means of a mirror on to a small thermo- 

 couple, or on to a metallic spiral. The instrument, therefore, 

 does not come into direct contact with the hot body. See 

 also Thermometer, Resistance, and Siemens's Water Pyro- 

 meter. [1, 2, 4, 5, 6, 13, 15, 16, 17, 20, 21, 24, 25, 26, 27, 

 28.] 



Pyrometer, Hobson's. A convenient hand pyrometer for testing 

 the temperature of the hot gases in iron furnaces. 



Pyrometer, Locomotive. An electrical pyrometer for use on 

 locomotives using superheaters. [4, 13, 16, 17.] 



Pyrometer, Optical. A type of radiation pyrometer employing 

 monochromatic luminous radiation. [4, 21, 26] 



Pyrometer, Radiation. A pyrometer operated by the radiant 

 heat emitted by the hot body. [4, 13, 16, 21, 26.] 



