392 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



mirror is attached by a slender rod of glass. For sensitiveness the disc 

 and mirror should be as light as possible. The suspension is hung by 



a fine quartz fibre so that 

 the disc makes an angle of 

 about 45° with the plane 

 of the coil. By the oscil- 

 lations in the receiving 

 circuit, which pass also 

 through the coil, oscilla- 

 tions are induced in the 

 disc, which is thus repelled 

 and tends to set itself at 

 right angles to the coil. 

 The vulcanite box, which 

 encloses the suspension, is 

 provided with a glass face 

 through which the deflec- 

 tions may be read by a 

 telescope and scale. The 

 period of the instrument 

 is five seconds, and the 

 suspension is so light that 

 its deflections are practi- 

 cally dead-beat, rendering 

 unnecessary the damping 

 magnet S N that was em- 

 ployed in the earlier ex- 

 periments. 

 When used in ordinary 

 wireless telegraph circuits this instrument shows great sensitiveness. 

 In making the present comparisons, the data are obtained not from 

 ordinary wireless telegraph circuits of the "open" type, but from circuits 

 consisting of closed loops. These circuits are of the same form as those 

 previously employed by Northrup, Reichmann, aud the author, and are 

 represented in Figure IV. The sending circuit A consists of a glass 

 condenser a in series with the spark-gap or interrupter b and a rectangu- 

 lar loop of wire four meters by three meters. About the spark-gap or 

 interrupter are counected the terminals of the secondary of a step-up 

 transformer, actuated by the alternating electric light circuit. The spark- 

 gap is made of small pieces of iridium set iu heavy brass balls. By 



Figure III. 



