Coefficient of Normal Viscosity of Metals. 119 



mirror m, and leaves time-marks every two seconds by the 

 action o£ an electromagnet p. The lens is so placed that the 

 images of the slits are formed on the cylinder, on which a 

 sensitive fim is wound. 



Fig. 2. 



fL~. P 



f 



fi S 



Fig. 3. 



In order to avoid the action o£ the diffused light on the 

 photographic film, the chronograph is covered with a wooden 

 box, on the upper side of which a small aperture is made 

 tor admitting the beam of light. As the source of light, an 

 electric arc of about 60 volts 10 amperes is used. 



Prepared for the photographing, the oscillation of the 

 suspended system is excited by air-puff, and the surrounding- 

 air evacuated by the rotary pump. When by virtue of the 

 internal friction, the amplitude of oscillation is reduced to 

 the desired magnitude, which can be observed through the 

 aperture, the chronograph is put in rotation, and the photo- 

 graph of the spots of light taken. 



The image of the slit S records on the film a regular sine 

 wave with the diminishing amplitude, while that of the 



