Electro-Magnetic Experiments, 331 



If the iron bar which so connects both poles of the magnet is sup- 

 ported by the hand, whilst the wires of the battery are removed from 

 one cup to another, the velocity with which the poles are reversed 

 may be actually felt by the duration of the impression of the weight 

 on the hand. This duration is certainly much less than one-tenth 

 part of a second. 



If we compare the velocity with which the pole? are changed, only 

 by transferring the wires from one cup to another, with the trouble 

 and time required to change the poles of a common magnet, capa- 

 ble of supporting seventy-six pounds, it becomes rather difficult to 

 bring the rule of Horace, Mil admirari, in actual practice. If the 

 poles of the horse-shoe are connected, not by the heavy iron rod, or 

 supporter, but by a slender steel needle, the poles may be reversed 

 before the needle by its weight overcomes the resistance of the air. 

 When the wires are shifted, a transient motion is perceptible in the 

 needle, but it does not fall. 



When the horse-shoe is loaded till the weight falls, its magnetic 

 force will be found considerably weaker, and some time must elapse 

 before it becomes again capable of carrying the same weight as be- 

 fore. It is well known that common horse-shoe magnets, when al- 

 lowed to drop their load, are considerably weakened, and often never 

 recover their former strength. 



In the experiments which I am now relating, the strongest action 

 of the horse-shoe takes place the instant when the trough is filled and 

 the connection is made. 



Although this horse-shoe is only possessed of a transient magnetic 

 force, the duration of which is limited to that of the galvanic action 

 of the battery, it is capable of communicating strong and lasting mag- 

 netism to hardened steel bars and compass needles. If a steel bar be 

 rubbed several times from end to end along the poles of the horse- 

 shoe, lasting magnetism is communicated to the bar, exactly as could 

 be effected by any other strong horse-shoe magnet. By touching 

 the horse-shoe in a contrary direction, the magnetism of the bar may 

 be destroyed, or the poles reversed at pleasure. In a similar way, 

 strong magnetism may be impregnated in compass needles, or their 

 poles reversed. 



It is a well known fact, that lightning often destroys the action of 

 compasses on ship board ; and there are even instances upon record 

 of serious accidents being occasioned by the poles of compass needles 

 being inverted by the electricity of lightning. Prudent navigators 



