IOWA ACADEMY OF SCIENCES. 75 



Sometimes, especially in warm weather, the composite sample will have floating 

 upon its surface flecks of milk-solids; these can be broken up, and the sample 

 brought into almost perfect mechanical condition, by means of a stiff test-tube 

 brush used as a pestle inside the jar, rubbing the flecks to pieces against the walls 

 of the latter. 



One must guard against error from the rising of minute flecks of milk-solids to 

 the surface while weighing out the charge; this is easily done by inverting the 

 weighing pipette once or twice just before running out the charge upon the 

 asbestos. 



It hardly needs saying that in summer the composite samples should be kept 

 in as cool a place as possible; ice or cold water would of course be useful. 



Experiment Station, Ames, loiva, Aug. 10, 1891. 



ON A NEW ASTATIC GALVANOMETER WITH A SINGLE SPIRAL 

 NEEDLE. 



[Abstract of paper read Dec. 27, 1887.1 



LAUNCELOT W. ANDREWS. 



Two types of astatic galvanometer are in common use. la one, two 

 needles with poles reversed are united to form a rigid system, in the other, 

 a single suspended needle is emploped, the directive force of the earth's 

 magnetism being compensated by an inmovable magnet suitably placed 

 north or south of the needle. (Hauy's method.) 



In both these forms the sensitiveness is very variable because a slight 

 change in the magnetic movement of either needle of the astatic combina- 

 tion, or of the fixed compensating magnet, exerts a disproportionately 

 great influence upon the sensitiveness of the instrument. 



It is, however, possible to construct a galvanometer not subject to this 

 disadvantage, having only one needle and no compensating magnet. 



A magnetic needle hung in such a way that a straight line passing through 

 its poles shall be parallel to its axis of suspension will experience no hori- 

 zontal directive force if placed in a uniform magnetic tield. It will be in a 

 word astatic. 



The author has utilized this principle in the construction of a galvanometer 

 of constant senstiveness, as follows: 



The needle is made in the form of a helix of one turn and is supeuded by a 

 cocoon fibre in such a way that the axis of the helix nearly coincides with 

 the axis of suspension. 



The extremities of this needle play freelj' within the cores of two coils of 

 insulated wire closely surrounding them. Its oscillations may be veiy effi- 

 ciently damped by winding the coils upon solid copper bobbins. 



The coils are advantageously so arranged, as in the instrument exhibited 

 before the Association, that they may be connected either in series or in 

 parallel circuit. 



