Short-Period Electrometer, 253 



(11) Measurement of Damping Factors, and other 

 Applications. 



In all cases the damping coefficient (x } as determined by 

 measuring the amplitudes of the curves, was greater than 

 R/2L. Thus with condenser II. and no secondary circnit, 

 /j, was 130, while R/2L was about 100. The presence of the 

 secondary coil increases \x which also depends to some 

 extent on the condenser, being rather less for condenser II. 

 than for any of the others. The various causes which might 

 affect the damping have not yet been enquired into, but one 

 or two results in this connexion may be mentioned. 



In one case condenser III. was in circuit with the coil A 

 with B open. From the curves /n was found to be 186. On 

 placing a non-inductive resistance of 10,000 ohms in series 

 with A, fju was increased to 264. The increase of fju was thus 

 78, while the increase of R/2L was 71*3. The additional 

 resistance therefore accounts for about 90 per cent, of the 

 increase in jju. 



In another case, with condenser I., coil A, and no secondary, 

 fju was found to be 142. On introducing a 1 mm. spark-gap 

 between platinum points into the circuit, jjl was increased to 

 234. The spark-gap thus appears to be equivalent in its 

 damping effect to a resistance of over 10,000 ohms. 



A number of photographs were taken for cases in which 

 the coil had an iron core. The periods were not determined 

 very accurately in these cases as, at the time when the 

 exposures were made, the importance was not realised of 

 photographing the spots in several positions on each plate. 

 The results illustrate, however, the great effect which a 

 secondary coil may have in these circumstances. In one 

 case, for example, a low-resistance coil wound on an iron 

 core was placed inside the coil A whose terminals were 

 connected to the condenser I. and to the electrometer. When 

 the circuit of the secondary coil was open the period was 

 about *00485 sec, giving the value 560 x 10 9 cm. for the 

 equivalent self-inductance calculated from the formula 

 T = 27Ta/ / LC. When the terminals of the secondary coil 

 were connected together the period was reduced to about 

 •00168 sec, and the equivalent self-inductance to 67 x 10 9 

 cm., i. e. rather less than the self-inductance of the coil A 

 alone without the iron core. The period would in this case 

 not be very different if the iron core were removed from the 

 secondary coil. 



Figs. 7 & 8. (PI. IX.) show curves for the secondary of 

 an induction-coil the primary of which was connected to an 



