132 



Lord Rayleigh and Dr. A. Schuster. 



[May 5, 



no doubt, is partly the reason why this forced vibration escaped their 

 notice. 



Air Currents. 



It has already been noticed that air currents in the box containing 

 the magnet affected its position to some extent, and we had to 

 investigate in how far our final results might be affected by this dis- 

 turbance. During the first night (December 2) our attention had 

 not been drawn so much as it was afterwards to the effect of these 

 air currents. We had previously ascertained, by a series of careful 

 measurements, that the rotation of the coil with open circuit did not 

 sensibly affect the zero position of the magnet, and we considered it 

 sufficient to note the zero as short a time as possible before each set of 

 four spinnings. The comparison of these zeros with the auxiliary 

 magnetometer showed that during the two hours of experimenting, 

 the needle had kept its zero within two divisions of the scale, so that 

 the changes during two successive spinnings (generally about five 

 minutes) must have been very small. On the second night 

 (December 6) however, the zeros were taken at the beginning and 

 end of each set of four spinnings, and the disturbance due to air 

 currents was found to be of more importance. The following table 

 reveals the fact that during a set of spinnings the magnet seems to 

 have moved in one direction, but that during the time the coil was at 

 rest and the comparison of resistance was made, it went in the opposite 

 direction. The numbers given are corrected for changes in the 

 direction of the earth's magnetic force as shown by the auxiliary 

 magnetometer. 



December 6. 



Number of teeth on 



Time. 



Position 



Approximate 



stationary circle. 



h. m. 



of rest. 



deflection. 



60 



8 53 



763-60 



218 





9 12 



766-35 





32 



9 31 



764-88 



397 





9 56 



765-78 





24 



10 9 .. 



762-67 



514 





10 38 



766-48 





Here, then, we have a gradual rise in the zero from one to over 

 three divisions during a set of four spinnings. The approximate 

 deflection is given in order to give an idea what amount of error the 

 uncertainty of the zero might introduce. 



Special experiments were now made, and it was found that by 

 placing a lamp about a foot and a-half from the magnet box, changes 

 amounting to eighteen divisions of the scale would be observed; 



