CRYOGENIC LABORATORY AT LEIDEN— GUILLIEN ^79 



of these cryostats and other special apparatus are constructed in the 

 laboratory. Kamerlingh Onnes established a school for the instruc- 

 tion of young artisans. They are selected by competition and pass 

 practical examinations from time to time. They receive instruction 

 in general theory, science, design, electrotechnics; and when they 

 leave the laboratory after a 5-year course, are employed in the 

 PTT, the University laboratories, the state industries, etc. The 

 facilities include two glass-blowing rooms, four precision instru- 

 ment shops, and an optical laboratory. In these work rooms the 

 laboratory employs about 80 young artisans in the construction 

 of cryostats, the liquefaction of gages, and other useful tasks under 

 the direction of 15 foremen. Thanks to these numerous trained 

 and experienced workers, most of the apparatus for the researches 

 is constructed and repaired at the laboratory, with resulting great 

 economy of time. A drafting force installed at the laboratory pre- 

 pares the working drawings for new apparatus, and the illustrations 

 for publication of the Communications of the laboratory. 



(c) Measurements. — The investigations nearly always require the 

 determination of at least one temperature. In general, the measure- 

 ment is made by means of an electrical resistance thermometer, con- 

 trolled by gas thermometry. To furnish heat during investigations 

 of bodies contained in Dewar flasks for studying their properties, 

 electrical current is supplied to measured resistances. For nearly all 

 researches involving electrical measurements, the cryostats in which 

 are enclosed the substances to be examined, are in connection with an 

 electrical testing room equipped with galvanometers and other ap- 

 paratus suited to making these measurements in conditions of pre- 

 cision and comfort. When it is necessary to measure simultaneously 

 several electric currents or resistances, the work is nearly always done 

 by several assistants in telephonic communication with the experi- 

 menter at the cryostat. Collaboration is also desirable, though in 

 somewhat less degree, when temperature measurements are associated 

 with the pressure of a saturated vapor as observed with the cathe- 

 tometer. For it is then necessary to hold the temperature constant 

 (or sometimes to cause it to vary in a chosen manner) during the 

 progress of the experiment. 



The low temperatures are produced in the 21 experiment rooms by 

 making a reduction of pressure of the appropriate liquefied gas with 

 which the cryostat walls have been filled. Three pumps maintain 

 pressures below 5 millimeters of mercury, available for the whole 

 laboratory. 



(d) Interpretation of observations. — A special computing room 

 equipped with calculating machines, tables of constants, tables of 

 functions, and other aids is at the disposal of the investigators. It is 



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