OF ELECTRO-DYNAMIC FORCES. 497 



composed of silver, and were heated to redness,) from the trans- 

 verse beam to the small rollers a a', was half a metre. 

 The price of the instrument is 10 guineas. 



Observations demonstrating the fundamental 

 PRINCIPLE OF Electro-dynamics. 



The following observations were not made with the instrument 

 which has just been described. However, it is unnecessary to 

 describe separately the instrument made use of on this occasion, 

 because it merely differs from the former in minor points of ar- 

 rangement, which were less convenient than those in the latter. 

 One important modification only requires to be mentioned, viz. 

 that the multipliei", which in the above description assumes an 

 invariable position, in which its centre coincides with the centre 

 of the bifilarly- suspended reel, was left moveable, so that it 

 could be placed in any position as regards the vibrating reel, for 

 the purpose of extending the observations to all relative positions 

 of the two galvanic conductors, which act upon each other. Now 

 as these two conductors form tw o coils, one of which can en- 

 close the other, and in the instrument described above the inner 

 and smaller coil was suspended by two threads, to serve as it 

 were as a galvanometer-needle, whilst the outer and larger coil 

 was fixed and formed the multiplier ; it was requisite for the ob- 

 ject in question to reverse the arrangement, and to suspend the 

 outer and larger coil by two threads so as to use the inner and 

 smaller coil as a multiplier, because it was only by this means 

 that the position of the multiplier could be altered at pleasure 

 without interfering with the bifilar suspension. It is at once 

 seen that the external reel, on account of its size, has a greater 

 momentum from inertia, which produces a longer duration of its 

 vibration ; this influence however may be easily compensated for 

 w hen necessary by altering the arrangement of the bifilar sus- 

 pension. 



As regards the observations themselves, it remains to be re- 

 marked, that to render the results comparable, the intensity of 

 the current transmitted by the two conductors of the dynamo- 

 meter was, simultaneously with the observation on the dynamo- 

 meter, accurately measured by a second observer with a galvano- 

 meter. This was requisite, because no reliance can be placed 

 upon the constancy of the intensity of the current during a con- 

 tinued scries of experiments, even when the so-called constant 

 battery of Grove or Bunsen is used. 



