46 



ELECTRICAL MEASUREMENTS 



tance. Cyclic phenomena are observed by the use of a revolving 

 mirror, as is customary. Permanent records may be obtained 

 photographically on a moving plate or film. A narrow slit is 

 placed immediately in front of the photographic surface and 

 behind a cylindrical lens so that the image of the fiber appears 

 on the sensitized surface as a shadow, which prevents the expo- 

 sure of the part of the surface on which it happens to fall. 

 This arrangement is used in physiological investigations. 



FIG. 18. Duddell thermo-galvanometer. 



The Duddell Thermo-galvanometer. The essential features 

 of this instrument, which is based on the radio-micrometer of 

 C. Vernon Boys, are indicated in Fig. 18. 



A single loop of silver wire having a high conductivity is sus- 

 pended by a quartz fiber in the strong magnetic field due to a 

 ermanent magnet. At the lower end of the loop there is a 

 ismuth-antimony thermo-couple and beneath this and as near 



