382 



PROCEEDINGS OF THE AMERICAN ACADEMY 



This table shows that the strength of the induced currents was 

 increased one hundred per cent bj the application of the armature. 

 In the above experiments the armature rested upon the limbs of the 

 electro-magnet directly at the poles of the electro-magnet ; and the ends 

 of the limbs of the horseshoe-shaped core were filed plane, so that the 

 armature rested completely upon them. The armature should have a 

 length equal to the distance between the poles of the electro-magnet. 



The above experiments were then repeated with cores made of small 

 iron wire, tied in bundles, v/hich were horseshoe in form, in order to 

 determine whether a core of this nature differed from a solid one. 

 The preceding results were sustained. Some difficulty was met in 

 filinor the ends of the bundle of iron wires, so that the armature 

 should rest completely upon them, but this was a mechanical diffi- 

 culty only. I nyxt proceeded to experiment on a larger scale. The 

 primary circuit was wound upon one limb of a horseshoe-shaped 

 core, the limbs of which were 26 cm. long, 4 cm. in diameter, and 

 the centres of which were 19 cm. apart. The primary coil con- 

 sisted of four turns of thick copper wire, having a total resistance of 

 .10 of an ohm. This covered one limb of the horseshoe uniformly; 

 the other limb was not covered. The secondary coil had a resistance 

 of 6,000 ohms, and the height of the secondary coil was equal to that 

 of the primary. A condenser was placed in the primary circuit, which 

 was also provided with an interrupter or break-piece. At first I en- 

 deavored to ascertain by measuring the length of the spark, produced 

 by breaking the primary circuit, the advantage of placing an armature 

 upon the poles of the electro-magnet. Very contradictory results were 

 obtained ; and at first sight it did not appear that any advantage 

 resulted from tlie use of the armature. I next, having drawn the 

 terminals of the secondary coil apart, so that a S23ark could just leap 

 across the interval, counted the number of sparks with breaks of the 

 primary circuit one second apart. The following table gives a series 

 of results: — 



