RECENT PROGRESS IN PHYSICS. 



337 



r over a greater surface, it has a less density, and consequently a slower 

 I discharge, which is favorable to the production of magnetism. 

 [ 3. The order of the periods of decrease and increase, as well as that 

 fof the reversal of tlie magnetism, will he changed by an alteration in 

 'the secondary circuit, such as introducing wires of constantly increasing 

 length. 



If the scciindary circuit remains metallic as before, but interrupted at 

 one place, so that the current has to pass with a spark, a very remark- 

 able influence is observed on the magnetic effect; often the magnetism 

 is in this way increased very greatly, sometimes it is weakened, and 

 again it is changed in direction. The strongest magnetization by the 

 secondary current, amounting nearly to saturation of the needle, has 

 been obtained in this manner. 



4. A continued change in the strength, as well as a change in the 

 direction of the magnetism produced by the secondary spiral, takes 

 place when, cceteris paribus, the length of the conducting circuit of the 

 main spiral is continually increased. 



The apparatus shown in figure 62 may be very conveniently used 

 in these experiments. The lower spiral may be taken for the secondary 

 circuit, and the magnetizing coil may be introduced between x and y 

 by screwing its ends in the clamps. 



§ 59. Production of heat by the secondary current. — It has already 

 been mentioned that the secondary current produces thermal phenom- 

 ena ; Riess lias also investigated thoroughly the laws of the development 

 of heat by the lateral current. — (Pog. Ann., XLVII, 65.) 



In the conducting circuit of the secondary spiral, a magnetic spiral 

 and an electrical air thermometer were introduced. The following 

 table contains the thermal and magnetic effects which the secondary 

 current produces when the surface and charge of the battery are 

 .changed. aS and q have the same signification's before. 



22 8 



