330 TENTH ANNUAL REPORT OF 



obtained by a series of very simple experiraents. It is perfectly 

 evident that this factor holds good for only a special rheometer, 

 and for that special instrument only as long as the experiment is made 

 in the same place. For instance, if the compass were removed from 

 Freiburg to Marburg, the reducing factor would receive another value, 

 because the horizontal intensity of the earth's magnetism is less in 

 Marburg, and thus a current producing less detonating gas, would 

 still produce a deflection of 45°. 



The above series of observations also present us with a proof that 

 Weber's tangent-compass can be used for determining the current 

 force in absolute measure only when its diameter is not much less 

 than 40 centimetres, (the length of the needle being three centime- 

 ters.) According to formula 4, the force of a current is propor- 

 tional to the radius of the ring, the angles of deflection of the 

 tangent compass being equal. The currents which produce a deflec- 

 tion of 45° in the two compasses above mentioned, are to each other 

 in the proportion of 38 to 30. The quotient of these diameters is 

 1.2666, while the quotient of the corresponding forces of the current 



is H = 1.0937. 



Having determined the reducing factor of a large tangent-compass 

 by accurate experiments, we can compute from it the horizontal intensity 

 of the earth's magnetism at the place of observation. The current 

 which produces a deflection of 45° in our large compass, (380 milli- 

 metres in diameter,) has, in chemical measure, the force of "70 ; in 

 absolute measure the force is, 



_ T : 190 



^ 2 : 3.14 

 But chemical measure is to the absolute measure as 1 . 0477 : 1 ; there- 

 fore in absolute measure this current has the value t.Utt = 66.813 ; 

 and we have, 



T : 190 



66.813 

 Hence, 



,14 



T = 2.2083. 



According to the chart the value of T at Freiburg is 2.21, which 

 accords very well with that computed above. 



To determine the quantity of chemical effect which a current pro- 

 duces, we might, instead of measuring the quantity by the volume of 

 explosive gas evolved, determine the quantity by weight of water 

 decomposed, as Kesselman has done, (tJber die galvanische Kohlen- 

 zink Kette,) and from that compute the volume of gas evolved. This 

 method of observing is susceptible of great accuracy, and it is to be 

 recommended on that account to those having an accurate balance at 

 command. The experiments given above prove that the direct meas- 

 urement of the volume of gas also yields very accurate results. 



§ 9. Resistance of the element. — The force of current of a galvanic 

 combination can be measured directly by means of a rheometer, and 

 reduced in accordance with the principles stated above, to a determi- 

 nate unit, for which the chemical unit is preferable on account 



