198 Ocean Magnetic Observations, 1905-16 



duced, this was not done. Care was used, however, to set the brushes very closely to eliminate 

 the necessity, for practical purposes, of this adjustment. The vertical circle is 10.2 cm. 

 in diameter, with a least graduation of 30 minutes of arc, and may be read directly, by two 

 fixed verniers, to 1 minute and, by estimation, to one-quarter of a minute. Suitable 

 means for clamping vertical circle and for slow motion are pro\aded. 



The bearings of the rotation axis of the coil are of brass, being V-shaped in longitudinal 

 section and running in agate cups burnished in the brass centering supports in the support- 

 ing ring. The coil is held in place by two U-shaped pieces of brass which carry the bearing 

 ends of the rotation axis. At one end of the rotation axis is mounted the commutator and 

 at the other end is a miter gear for use in the rotation of the coil. The spool of the coil 

 is made of hard rubber^ of 24 mm. outside thickness and of 74 mm. outside diameter. The 

 inside diameter of the winding of the coil is 26 mm., the outside diameter, 73 mm., and the 

 width of winding, 17.5 mm. There ai'e 65 layers (3,162 turns) of double-silk-wound 

 magnet wire No. 30 B. & S. gage, with a double thickness of paper at every fifth layer. 

 For protection against moisture conditions encountered on board ship and against possible 

 abrasion and short-circuiting of the turns, the outer surface of the coil and its connections 

 are heavily coated with paraffin. The resistance of the coil is about 175 ohms. 



The instrument has been very carefully balanced by the use of counterweights attached 

 (as has been found necessary) to the parts which may take up different positions at different 

 times of observation; e. g., the coil in its mounting has been carefully balanced around the 

 axis supporting the bearing ring, and the whole has been balanced about the center fine 

 of the spindle bearing. Thus, when the instrmnent is mounted, the gimbal ring is supposed 

 to remain level for any orientation or po.sition of any part of the instrmnent. 



For the purpose of determining the magnetic meridian, a sighting telescope and a 

 compass are provided, suitable mounting wyes being placed so that the Une of sight or the 

 magnetic axis of the needle will be in the vertical plane through the rotation axis of the coil. 

 The magnetic meridian may thus be determined by sighting upon marks of known magnetic 

 bearing or by actual observation of the compass. Parallax in the compass readings is 

 avoided with the aid of mirrors mounted immediately below the ends of the needle. 



The gearing for rotating the coil of the earth inductor is self-contained, and is shown 

 clearly in Plate 14, Figure 5. A hole has been drilled through the center of the spindle and 

 a shaft mounted in it with a miter gear at the upper end in suitable bearing ; this engages 

 a second miter gear mounted on a shaft set at about 45° from the vertical in fixed bearings 

 on the standard frame. A third gear at the upper end of the inclined shaft engages a similar 

 miter gear attached to an axle rotating in the center of the horizontal bearing-end of the 

 supporting ring. Inside of the supporting ring there is attached to this axle a gear of 

 102 teeth mounted in a hollow spherical frame which permits the coil to turn freely inside 

 it, and which engages a gear of 25 teeth attached to the rotation axis of the coil. 



The method of transmission of the rotary motion without disturbing the gimbal rings 

 is as follows: At the point of intersection of the two diameters tlirough the supporting 

 knife-edges of the two gimbal rings a very small universal joint has been mounted; this is 

 made so as to be adjustable vertically and has sufficient lost motion between its two parts 

 to allow for the very shght play between the two rings in their bearings. The motion is 

 transmitted to this universal joint by means of two gears, shafts, and a handle (see Plate 14, 

 Fig. 5) carried in a frame attached rigidly in a diameter of the reversible bearing ring of the 

 gimbal. By means of a slight inclination of the support carrying the transmitting shaft, 

 it is possible to rotate the operating crank in any position of the ring without interference 

 from the supporting frame. A cross-pin in the lower end of the rod through the spindle 



'Tliis material was used instead of brass because of the greater mechanical ease of maintaining insulation and eliminating 



indui-tion effects. 



