358 EECENT PROGRESS IN PHYSICS. 



7. The insertion of iron wires yields remarkable phenomena, pro- 

 ducing anomalous periods of very considerable strength. Hankel 

 found them particularly striking with thick, long iron wires. While 

 a thick copper wire greatly weakens the magnetism, the latter is con- 

 siderably streagthened by a thick iron wire. On introducing an iron 

 wire 1.27 millimetre diameter and 131 metres longit gave, forinstance, 

 the result for a charge 6, a normal maximum 11 ; for a charge 36, an 

 anomalous magnetism of the strength 9|, taking for unity the mag- 

 netizing strength adopted in constructing the above curves. 



§ 71. Leyden jars of thick glass. — Winter, of Vienna, constructs 

 Leyden jars which have a much greater striking distance than those' 

 in general use, and he accomplishes this by using vessels with very 

 thick sides, (over 1 line,) and by leaving a very wide uncoated border. 

 Spontaneous discharge is prevented by the width of the uncoatedi 

 border, and perforation of the glass is prevented by its thickness. In. 

 such jars the tension of the free electricity on the inner coating cam 

 reach a far higher degree than in the ordinary thin jars, in which, if 

 a spontaneous discharge does not occur, a fracture of the glass is to 

 be feared. 



The mutual induction of opposite electricities of the two coatings,; 

 in consequence of the great thickness of the glass, is less perfect than 

 with thinner glass. With the same quantity of coating, and with 

 the same density of the free electricity on the inner coating, less elec- 

 tricity will be accumulated in thick glass jars than in those of thin 

 glass ; in general, therefore, the quantity of electricity which a thicb 

 glass jar can receive is less, but the tension of the free electricity on 

 the inner coating, and consequently the striking distance, is greater.' 

 It is to be expected that with the greater striking distance, other 

 effects of the discharge will also suffer a change. All effects of the 

 discharge stroke, in which it is chiefly desirable that a great quantity 

 of electricity should be sent through a body, can be produced better 

 with large, thin glass jars, but where the force of the shock is the 

 main object, thick glass jars serve the purpose better; hence it ap- 

 peared to me probable that the perforation of glass plates should take! 

 place much more easily with thick jars than with ordinary thin ones. 

 Trial perfectly sustained my supposition. Formerly, in using large,, 

 thin jars, a great number of revolutions of the machine were necessary 

 to charge the battery sufficiently for the perforation of glass, and eveci 

 then the experiment did not always succeed satisfactorily ; now, 2C 

 revolutions of a very moderate electrical machine suffice to charge a 

 thick glass jar so as to produce this effect with certainty. 



Fi?. 71. The thickness of the glass jar, fig. 71, is about 1 line 



each coating has a surface of about 9 square decimetres, 

 and the uncoated border is 22 centimetres in height. 



I have not studied carefully the influence of the thick- 

 ness of the glass upon the effects of the discharge stroke. 

 and only make this notice in order to draw the attentioD 

 of other physicists to the point. It is much to be wishec 

 that Eiess would take up this subject, since he has already 

 _ labored in this field with such generally acknowledgec 

 good results. 



