The Duchemin Compass. "The ordinary compass needle offers to the two magnetic currents of the earth, a surface merely narrow and longitudinal. Reason tells us that the greater the surface which the compass offers to the immense action of terrestrial magnetism, the greater and more positive will (other things being equal) be the resultant effect. This thought has been on my part the object of long meditations, and I have had the idea of creating, by utilizing the artificial currents which we possess, and by conceiving a new process of magnetization, the magnetized circular bar or circle with two magnetic poles at the extremity of the same diameter. This magnet possesses two neutral lines, which give a complete surety against the displacement of the magnetism; the figure obtained by means of iron filings placed above the magnetized ring, attests the perfect distinction of the magnetism in the circle.
From this idea has arisen the compass with a circular needle, whose conception was at first received with marks of profound incredulity and irony.
Duchemin's Compass with two concentric circular needles and steel traverse connecting the poles. The maximum magnetization is at the North and South points respectively, decreasing to zero at East and West, as shown by the shading upon the circles.
The circle, magnetized by a special process which gives magnetic stability, placed upon a pivot, or suspended by a thread from its center, constitutes a true compass, the north pole pointing to the south, the south pole to the north. The poles being considerably farther apart than in the ordinary straight needle, the terrestrial attraction is consequently multiplied. Further, the circle forming a symmetrical figure about the point of suspension, its mechanical stability is greater; the mass acted upon by terrestrial magnetism being thus augmented, it is clear that the oscillation of the nautical circular compass is less uneasy under the blows of the sea, than that of the ordinary straight needle. From these different points of view, the circular compass is deemed as a true step in advance, which the trials at sea have fully confirmed.
To give an idea of the magnetic force of this instrument, (a force which can be augmented by the addition of concentric circles, and of a traverse of steel, itself magnetized and forming an armature) I will add that a circular compass of 20dem in diameter, caused in an ordinary needle a deviation of from 450 to 700; under the same conditions the ordinary needle caused a deviation of but 17° or 20°.
Experiments on Board the Faon.
These were divided into two series: first those upon the ordinary compass designed to ascertain the value of the circular compass when placed under the same circumstances as the former; second, those looking to the correction of local influences by the addition of a movable concentric magnetized circle. For the first series an ordinary mariner's compass, having a needle 20dem in length, previously magnetized to saturation, was compared with a magnetized circle of the same exterior diameter; the two compasses were placed under the same conditions as to pivot and agate. Following are some extracts from the report.
'Sensibility. The sensibility of the circular compass leaves nothing to be desired: it is superior to that of the ordinary needle. Removed from the magnetic meridian, the circle returns more quickly than the needle, although the friction of the first is greater, since the weight of the entire system is one hundred and forty-one grammes, while that of the ordinary card is but sixty two. It may easily be conceived that it may be possible to store up in a circle more magnetism than in a bar, and that with equality of weight. By simple inspection of a magnetized circle this may be comprehended; besides a needle magnetized to saturation allows the fluid to escape at the ends, if it is desired to charge it more: it is not the same with a circle.'
The report establishes the fact that the concentric circle augments the sensibility of the circular compass while giving it additional direct lye force. To my mind this circle has another advantage; it seems to correct the time or the length of the oscillations, that is to say, if a compass with one circle gives a period of four minutes of oscillation, the second circle will by its presence diminish about one-half the duration of this period. I can say with the report, that during all our passage, with a smooth as well as with a rough sea, we have never found the circular compass asleep; and that the helmsmen have been able to detect the slightest variation of the course by a simple inspection of the card.
'Mechanical Stability. The oscillations of Up circular compass about its position of equilibrium, are less than those of the ordinary card; this may be seen, a priori, by reason of the equality of the moment of inertia in every respect.'
With the ordinary compass this equality is obtained by utilizing additional weights, or by means of a complication of bars which augments the weight of the instrument: but is this not to the detriment of the directive force?
In the circular compass this equality of inertia exists of itself, and the parts of the circle which contribute to its mechanical stability contribute also to its directive force: this fact is incontestable. I find its proof in the employment of a plated brass circle by certain constructors: this circle is not, in the opinion of sailors, applied to the compass without great detriment to its directive force.
Evidently, like the ordinary compass, the circular compass oscillates with the rolling of the vessel; but its oscillations are slow and are not to be compared with those of the former. The vessel rolling greatly in a heavy sea, the helmsmen take their bearings without any uneasy movements of the compass.
In a smooth sea, the liquid compass sleeps in a lazy manner, while the circular compass has a stability almost equal to that of the other, and a sensibility very superior to it.
Magnetic Stability. The stability of the circular compass is very satisfactory: drawn from the magnetic meridian on account of the rolling of the vessel (when the masses of iron on board acquire a polarity ever changing with the roll) the number of its oscillations is much less than that of the ordinary compass: like the light card, for smooth weather it is excellent, for it does not sleep: like the heavy card, for rough weather, it presents a mechanical stability which should lead to its preference over the common compass.'
Coincidence of the axes. In his statement, the author of the report from which these extracts are taken, asks if the inventor does not experience serious difficulties in localizing the magnetism at the desired points, where the constructor indicates by marks the north and south points of the circle. The magnetization of the circle is very simple: it is performed instantaneously by means of a bar of soft iron designed by myself and constructed by M. Ruhmkorff, who has lent me the most assiduous aid and without whose skill it would have been very difficult for me to establish my circular compasses. Hence I affirm there is no difficulty on this point.
'It is conceivable that there is no definite reason to fear that the poles will be displaced, if it be considered that there exist two neutral points whence the magnetism begins at zero and is concentrated at the poles of maximum strength. There exists no reason why these neutral points should become magnetized, and hence the stability of the maximum poles.'
The report goes on to say that I have presented two kinds of circular compasses, in one of which the steel ring is supported by a traverse of aluminum, while in the other the traverse is of magnetized steel. The fact is recognized that both these compasses function perfectly: but the opinion is expressed that it would be better to adopt the steel traverse, whose poles coincide with those of the circle of the same name, for it is natural to think that the polarization of the circle will be maintained in preference to the points where the trace of the magnetic axis of the needle cuts the ring. Thus the armature system seems to have the greater weight with the author of the report.
But, in order to go farther than the objection which has been made, I have placed above the point of suspension a small magnetized needle with poles reversed. The axis of suspension of this small needle coincides with that of the circle; its magnetic axis naturally takes its place in the direction of the diameter of the poles and follows it invariably with remarkable energy and fidelity. If the magnetic axis, or rather the magnetic diameter, of the circle does not coincide with the axis of figure which ought to pass through the north and south points marked upon the card, the small needle will indicate it. One will always be warned against an accident, which is less to be feared with the circle than with the needle.
'To sum up, the circular compass is an instrument worthy of all attention from the navigator. By perfecting its practical construction and by placing the circles under a card of isinglass, we shall have an instrument, sensible and stable and one which will constitute a true step in advance. There is the very desirable power of augmenting its magnetic stability and sensibility by the addition of concentric circles without changing the equality of the moment of inertia in any respect, and without fearing (as in the compass with several needles) the influence of the neighboring poles, which has a tendency to destroy the magnetism.'
The idea of correcting the deviation of the compass by the displacement of one of the concentric circles of the circular compass appears to me to be untenable; for, from observations made at sea, I am persuaded that the cause of the phenomena which it is necessary to overcome being due to divers exterior actions, any interior disposition of the instrument, however ingenious it may be, will prove insufficient. Experience has shown that the circle behaves like the straight needle. Nevertheless, the circular compass has some advantage over the ordinary one, as the report recognizes, with regard to its deviations, on account of their greater regularity; its greater magnetic energy making its indications less subject to accidental errors. As the poles are placed symmetrically about the point of suspension and are separated by a greater extent than in the straight needle, the terrestrial action is consequently multiplied. By displacing the interior movable circle about its axis there is obtained, it is true, an action which corrects for a single course, but at the same time there is created so to speak a new instrument, whose magnetic direction is the resultant of the action of the two contrary poles. This new instrument is submitted to local deviations which are not sensibly lessened.
'By swinging the vessel at the compass buoys in Cherbourg harbor the circular compass has enabled us to detect errors in the construction of the deviation table, errors due to the want of energy and precision of the liquid compass. Upon this last point the circular compass leaves nothing to be desired.'
Experiments on Board the Ironclad Savoie.
The Duchemin compass has a magnetic power much more considerable for the same mass than one of our ordinary compasses. This power is from two to four times greater than that of those which have been experimentally compared with it. It has then over our own compass the advantage of much greater sensibility.'
This report suggests the application of the circular needle in the liquid compass.
Such, concisely stated, are the results which the new compass has given at sea. I will add that the personal study which I have made of the compass with circular needles leads me to believe that the extreme sensibility which can be given to this instrument might be advantageously utilized in our meteorological observatories.
Experiments on Board the Duchaffaut.
'The magnetic compass of M. Duchemin has a much greater magnetic power than the regulation compass which we have on board, which makes it more fixed and causes its local deviations to be less than those of the other.
The results obtained on board this vessel by comparing the two compasses, placed successively in the same binnacle and under the influence of the same causes of deviation, are in favor of the Duchemin compass. Compared with a liquid compass it is less mobile and its relative steadiness gives more facility in taking bearings at sea.
I find your compass less mobile than those of the ordinary pattern; that is to say, that it is deranged less upon any given course because it has a greater magnetic attraction and because its poles remain in the magnetic meridian with greater persistence than those of the ordinary ones.'
Experiments on Board the Transport Orne,
with a new circular liquid compass, during a voyage of circumnavigation.
By an order of the 26th and 29th of May, 1875, the Minister of Marino directed comparative trials of different compasses to be made aboard the Orne.
The compasses experimented with were,
Ritchie's American compass with two needles and floating circle.
Duchemin's circular liquid compass.
Regulation Azimuth compass with single needle.
Regulation do do four needles (light card.)
do do do do (heavy card.)
do Liquid compass with two needles.
The Dumoulin-Troment liquid compass with card, mounted with a Laurent circle.
The Dumoulin-Troment liquid compass with two needles in lozenge with additional segments in brass.
The programme for the trial of the Duchemin compass indicated three points to be observed; fixity of the line of the poles; sensibility; mechanical stability.
Fixity of the line of the poles. The Duchemin compass observed during the whole voyage concurrently with the various others always showed the greatest stability of magnetism: on the night of the 23 August, 1875, during which a violent storm raged unceasingly, a thunderbolt fell at some distance from the vessel, at the same time that a brilliant St. Elmo's fire at the peak of the spanker gaff attested the highly electric state of the surrounding atmosphere. Experiments made the succeeding day, demonstrated that the Duchemin compass had not in any way been influenced by the electricity in its vicinity. Observed daily, while by the vessel running perpendicular to the isoclinic lines it was subjected to continual changes in its induced magnetism, this compass showed no displacement of the line of its poles.
Upon arriving at New Caledonia, different articles of iron (boilers, pieces of machinery, etc.,) were landed: other articles (guns, etc.,) were embarked and placed in different positions from those the others had occupied.
The compass was submitted to violent shocks; among others a fall which broke one of the gimbals; it was frequently removed from aft to the bridge, and was allowed to remain upon the bridge at 0.m5 from an iron stanchion, at 3.m from the galley funnel and at 2.m from the smoke stack which was sometimes up, sometimes down, sometimes hot and sometimes cold. In either case the fixity of the line of the poles showed no alteration.
Polar auroras produced not the least perturbation. In New Caledonia the Orne passed through the Wodin strait, a narrow passage between ferruginous mountains: the deviations shown by this compass were not different from those of the others. Finally, as opportunity offered, the deviation tables were examined as a verification of the fixity of the line of the poles. This compass has shown in its deviations some differences from those of the steering compass, but they are so small that they may be ascribed to errors of observation.
Sensibility. The Duchemin compass possesses this quality in the highest degree: in calm weather it shows rapidly and almost instantaneously the slightest deviation from the course. Its magnetic power is quite great; when the vessel changes her course, the card appears perfectly independent and turns without the least inclination to follow the vessel's head, either on account of the resistance of the liquid or the friction upon the pivot. In point of sensibility, this compass appears to be excellent; its qualities of stability are not less remarkable.
Stability. Whether the engines turn at a high or low speed no effect is visible upon this compass; there is no violent motion, no trembling. This property it has in common with other liquid compasses. When the vessel is moved by the sea in calm weather, that is to say when the rolling motion though considerable, is regular, the Duchemin compass follows the rolling with an extremely small retardation in its oscillations, and the movements of the card cease almost as soon as the rolling motion.
That is to say, on board the Orne with a roll lasting from 5.5 seconds to 6 seconds, this compass has its line of poles carried successively to starboard and port in the same interval of time, but without any accumulating impulse, and this whatever be the heading of the vessel. In this way the amplitude of the oscillations due to the rolling are always restrained and rapidly arrested, which proves at the same time the great magnetic power as well as the excellent stability of this compass. In bad weather, when the vessel is violently tossed, the movements sharp, the recovery quick, the pitching violent, the Duchemin compass shows excellent qualities. Sometimes, but rarely, the card surprised by a sudden movement of the vessel takes up a notable jumping motion, but it never becomes greatly excited: its movements soon cease and it takes up rapidly its mean position and feeble motions. Sometimes, in a heavy sea, when it was impossible to take .the bearing of the sun with the ordinary azimuth compass with a single needle or even with the heavy card with four needles, the Duchemin compass was successfully used.
To sum up, this compass, on account of its great magnetic power, by reason also of the property which it possesses of having the same moment of inertia upon all courses, is less affected than any other by different disturbing causes. All questions of detail apart, the Board is of the opinion that among all the compasses experimented with, the Duchemin compass, thanks to its superior magnetic power, appears to be the most serviceable.'
On board the Dupleix, during a voyage to Iceland, experiments were made and the Board reported in much the same manner as the officers of the Orne; they close their report by saying, that
'They are of the opinion, that under all circumstances of wind and sea, whether under sail or under steam, the Duchemin circular compass is superior to the ordinary compasses in use. In particular they have appreciated its sensibility in the northern regions which the Dupleix has visited during the voyage, regions in which the ordinary compass has a very decided tendency to insensibility.'
On board the Laplace, in 1876,
‘On the 4th of May while lying to in a gale, and on the 11th of October, while scudding, the amplitude of the oscillations of the ordinary compass exceeded a point and a half; while the oscillations of the Duchemin compass were barely more than a quarter of a point, which permitted a course to be kept, a thing impossible to be done with the ordinary compass.’
On board the iron-clad Atalante, during a cruise in the Pacific,
'The Duchemin compass showed itself always superior to the binnacle compass with which it was compared; its oscillations have never exceeded two degrees, while those of its neighbor were as great as eight or ten degrees.'
The compass on board the Duchaffaut, (previously spoken of) was one of the first constructed, and was afterwards placed on board the Correze, and was highly commended in the report of the Board which was ordered to experiment with it.
The liquid compass which was on board the Orne was afterward placed on board the Comnao, and her commanding officer was as much pleased with it as were the officers of the first named vessel.
‘On board the ironclad Richelieu there were two Duchemin compasses. One was an azimuth compass, placed in the forward conning tower, under the eye of the officers of the watch, about 3m from the smokestack, and 2m from the electric apparatus which shows the course at night; 2.m5 below and 2m one side are situated two turrets each containing a 15 ton steel gun. The other was a binnacle compass placed in the tower near the steering wheel, a little forward of the first one and about 4m from it, and about 1m from the midship line. These two compasses are more sensitive than any of the others, notwithstanding the vicinity of these great masses of Iron.’"