202 Dr. Tyndall on the Progress of the Physical Sciences : 



excited. But the quantity that remained was very small; so 

 small, indeed, that the magnetic needle, which was placed 600 

 millims. distant, deviated under the influence of this remaining 

 magnetism from the magnetic meridian scarcely one-tenth of a 

 degree. 



The following are the dimensions of our iron cylinders : — 



No. 1. 108 millims. long and 55 millims. diameter. 



No. 2. 131 ... 38 



No. 3. 131 ... 19 



No. 4. 200 ... 9 



The spiral which belonged to No. 1 was 86 millims. in length, 

 and consisted of 90 windings ; 8 windings foimd space on every 

 10 millims. The thickness of the wire was 4 millims. 



For Nos. 2, 3 and 4, a spiral 110 millims. long and of 560 

 windings was used. In this case, therefore, we have about 43 

 windings to every 10 millims. of the core. The thickness of the 

 wire was 2 millims. 



The tangent instrument had two rings. According as the 

 greater or the less was used, the tangent of the angle which the 

 current caused the needle to make with the magnetic meridian 

 must be multiplied by 31*62, or by 63"24, to reduce it to the 

 unit chosen by Miiller. 



Finally, to reduce the magnetizing force to the same unit, the 

 force of the current must be multiplied by 8 or by 43, according 

 as the bar No. 1, or one of the three remaining bars, was used. 



A glance at the following table will show that the magnetizing 

 forces used by us, not only far exceed those of Miiller in absolute 

 value, but that they were also more widely varied. Our weakest 

 stream was 3'3, our strongest 231*6. 



The letters at the head of the table have the following signi- 

 fications : — 



expresses the angle which the tangent needle, when acted 

 upon by the stream, makes with the magnetic meridian. 



<!>', the angle which the small needle, acted upon by the mag- 

 net, makes with the magnetic meridian. 



(j>", the angle due to the action of the helix with no core 

 within it. 



The numbers contained in the columns <})' and (f)" are not those 

 directly read off, but are the mean of the readings right and left 

 of the needle's position of equilibrium. 



K expresses the calculated magnetizing force. 



i/r, the angle due to the action of the iron core alone. 



The rest is evident without further explanation. 



