RESEARCH ON THE DETERMINATION OF THE WAVELENGTHS OF HEAT RAYS AT LOW TEMPERATURES. Pgs. 1-5

EUVRES DE P. CURIE.

RECHERCHES SUR

LA DETERMINATION DES LONGUEURS D’ONDE

DES RAYONS CALORIFIQUES A BASSE TEMPERATURE.

En commun avec P. DESAINS.

Comptes rendus de V Academie des Sciences , t. XC, p. i5o6,

seance du 28 juin 1880.

WORKS OF P. CURIE.

RESEARCH ON

THE DETERMINATION OF THE WAVELENGTHS

OF HEAT RAYS AT LOW TEMPERATURES.

In collaboration with P. DESAINS.

Proceedings of the French Academy of Sciences, vol. XC, p. 1506,

session of June 28, 1880.

Dans Line serie de recherches recentes, M. Mouton a fait connaitre one methode par laquelle on peut determiner avec beaucoup de precision ies longueurs d’onde des rayons calorifiques obscurs, et il a etudie les relations qui existent entre ces longueurs d’onde et les indices de la refraction que les rayons qu’elles caracterisent eprouvent a travers differentes substances, ie flint, le crown et le sel gemme.

In a series of recent studies, Mr. Mouton has described a method by which the wavelengths of dark heat rays [infrared] can be determined with great precision, and he has investigated the relationships between these wavelengths and the indices of refraction that the rays they characterize exhibit when passing through various substances, namely flint, crown [glass lens], and rock salt.

La methode suivie par M. Mouton suppose que les rayons sont transmis a travers des polariseurs et des analyseurs, et jusqu’ici les seuls polariseurs 011 analyseurs qui aient paru propres a ses experiences ne sont en aucune facon permeables a la chaleur venant de sources qui n’ont pas une tres haute temperature.

The method employed by Mr. Mouton involves passing the rays through polarizers and analyzers, and so far the only polarizers and analyzers that have proved suitable for his experiments are in no way permeable to heat from sources that do not have a very high temperature.

Dans ce cas special nous avons cherche a resoudre le probleme par un emploi convenable des reseanx de Fraunhofer, el nous demandons a l’Academie la permission de Ini soumettre nos res u l tats.

In this particular case, we sought to solve the problem through the appropriate use of Fraunhofer gratings, and we hereby request permission from the Academy to submit our results.

Le reseau que nous avons le plus souvent employe etait une nappe de Ills metalliques de 1/8 de millimetre de diametre. 11s etaient tendus parallelement entre eui sur un cadre resistant et a des dis¬ tances sensiblement egales aussi a 1/8 de millimetre, de telle sorte que V element optique du reseau avait une longueur egale a ¼ de millimetre, ou plutot, d’apres ^observation directe, a o"",252. Etudie optiquement, ce reseau a laisse pen de chose a desirer, et, en l’employant a determiner la longueur d o nde de la lumiere du sodium, nous avons obtenu les resultats ordinaires.

The grating we most frequently used was a sheet of metal wires 1/8 mm in diameter. They were stretched parallel to one another on a sturdy frame and at substantially equal distances of 1/8 millimeters, such that the optical element of the grating had a length equal to 1/4 millimeters, or rather, according to direct observation, to 0mm, 252. When examined optically, this grating left little to be desired, and, using it to determine the wavelength of sodium light, we obtained the expected results.

Pour operer avec ce reseau, nous le placions a 0.5m environ d’une fente par laquelle passait un rayon de chaleur obscure, sen¬ siblement homogene, dont la direction etait perpendiculaire a celle du reseau. Immediatement contre celui-ei et du cote de la fente etait une lentil le de sel gemme d’environ 0.25m de foyer. L’image calorifique de la fente se faisait de bautre cote de la lentille, a une distance voisine de 0.5m, et dont la valeur rigoureuse fjtait calculee d’apres la connaissance des indices des rayons employes.

To operate this grating, we placed it approximately 0.5m from a slit through which passed a beam of diffuse, roughly uniform heat, whose direction was perpendicular to that of the grating. Immediately adjacent to the grating and on the side of the slit was a rock salt lens with a focal length of about 0.25m. The thermal image of the slit formed on the other side of the lens, at a distance of approximately 0.50 m, and its exact value was calculated based on the known indices of the rays used.

En ce point et perpendiculairement an rayon central, on placait une regie di\isee, le long de laquelle pouvait se mouvoir une pile thermo-electrique dont les deplacements pouvaient se mesurer a 1/10 de millimetre pres (1).

At this point and perpendicular to the central ray, a divided scale was placed, along which a thermoelectric cell which could move, and whose displacements could be measured to within 1/10 a millimeter (1).

(1) Quand la pile etait placee de facon a recevoir le rayon central lui-meme, Teffet thermoscopique produit etait maximum et, en general, considerable. II diminuait. rapidement des qu’on ecartait la pile de cette position dans un sens ou dans 1’autre. Bientot l’intensite de Taction atteignait un minimum qui souvent n’avait d’autre valeur que zero; puis, en continuant le mouvement toujours dans le meme sens, on atteignait un nouveau maximum, dont la valeur atteignait environ le cinquieme de Tintensite du rayon central. La pile etait alors en coin¬ cidence avec le premier spectre. En continuant a Teloigner de Timage centrale, nous avons plus d’une fois ti-ouve un second minimum et un second spectre. Dans tons les cas, le ph^nomene s’est toujours montre svmetrique par rapport au rayon central.

(1) When the cell was positioned to receive the central ray itself, the thermoscopic effect produced was at its maximum and, in general, considerable. It decreased rapidly as soon as the cell was moved away from this position in either direction. Soon the intensity of the effect reached a minimum that was often equal to zero; then, continuing the movement in the same direction, a new maximum was reached, whose value was approximately one-fifth of the intensity of the central beam. The stack was then in coincidence with the first spectrum. By continuing to move away from the central image, we found more than once a second minimum and a second spectrum. In all cases, the phenomenon always proved to be symmetrical with respect to the central ray.

Ea fente de la pile et la fente d’admission avaient le plus sou¬ vent une largeur de 0.5mm ou de 1 mm; quelquefois nous avons porte cette largeur a 2mm. Ges variations n’ont jamais eu d’in-fluence que sur l’intensite absolue des maxima observes et nullement sur leur position.

The slit in the stack and the entrance slit were most often 0.5 mm or 1 mm wide; on some occasions, we increased this width to 2 mm. These variations affected only the absolute intensity of the observed maxima and had no effect whatsoever on their position.

La met bode que nous exposons suppose necessairement Femploi de rayons calorifiques homogenes, et, pour que les resultats aient une utilite scientifique, il faut preciser la position occupee dans le spectre par chacun des rayons employes.

On satisfait de la maniere suivante a cette double condition :

The method we describe necessarily requires the use of homogeneous heat rays, and, for the results to be of scientific value, it is necessary to specify the position occupied in the spectrum by each of the rays used.

This dual condition is satisfied as follows:

On commence par laire un spectre en prenant pour source une lampe de MM. Bourbouze et Wiesnegg, a dome de platine incan¬ descent, et un appareil refringent tout en sel gemme, dans lequel le prisme ait un angle bien connu, 60" par exemple. Puis, comme s’il s agissait d’etudier la distribution de la chaleur dans le spectre, on dispose, a l’endroit oil ee spectre est bien net, une pile dont le mouvement pent etre exactement mesure.

We begin by observing a spectrum using as a source a lamp by Messrs. Bourbouze and Wiesnegg, with a dome of incandescent platinum, and a refracting apparatus made entirely of rock salt, in which the prism has a well-known angle, 60" for example. Then, as if the aim were to study the distribution of light in the spectrum, a pile is placed at the point where the spectrum is very sharp, the movement of which can be measured exactly.

Alors on detache la pile de la plaque porte-fente contre laquelle elle est d’ordinaire fixee; mais cette plaque reste en place, attenante au pied a mouvement, et par suite la fente peut etre amenee successivement en toutes les regions du spectre et dans toutes ses positions : sa distance aux rayons de la llamme sodique peut, etre exactement mesuree. II est des lors toujours possible d’isoler a travers cette fente, un faisceau de rayons homogenes et de refrangibihte connue. II est entendu que, les choses ainsi disposees, on fixe le pied de la regie porte-fente et l’on ne deplace plus que la fente elle-meme. Dans la pratique, avant de separer la pile de la fente, il est bon de determiner la position exacte du maximum et la vale u r des intensi tes en quelques a litres points.

The stack is then detached from the slit-holding plate to which it is normally attached; however, this plate remains in place, attached to the movable base, and consequently the slit can be successively moved to all regions of the spectrum and into all its positions: its distance from the rays of the sodium flame can be measured with precision. It is therefore always possible to isolate, through this slit, a beam of homogeneous rays of known refractive index. It is understood that, with the setup arranged in this way, the base of the slit-holding mount is fixed, and only the slit itself is moved. In practice, before separating the slit from the stack, it is advisable to determine the exact position of the maximum and the intensity values at several points.

Dans le spectre produit comme nous l’avons indique plus haut, les rayons distants du jaune d’un angle egal a i^bo' n’etaient plus transmissibles a travers une lame de verre de om, oi d’epaisseur, et pourtant, sans prendre de fente de large ur superieure a om,ooi, nous avons pu aisement faire des determinations de longueurs d’onde sur des rayons dont la distance aux rayons jaunes atteignait a°43/, et nous avons trouve cette longueur egale a omm,oo56. Pour les rayons situes a 3° i6; de ceux de la raie D, la faiblesse de l intensite nous a forces a porter les largeurs des fentes a om,oo2; mais les minima n’en ont pas etc moins nettement accuses.

In the spectrum produced as we indicated above, rays distant from yellow by an angle equal to 1° 55’ were no longer transmissible through a glass plate of thickness 0.01 m thick; yet, without using a slit wider than 0.001 m, we were able to easily determine the wavelengths of rays whose distance from the yellow rays reached 2°43’, and we found this wavelength to be 0.0056 mm. For the rays located 3° 16' from those of the D line, the low intensity forced us to set the slit widths to 0.002 m; nevertheless, the minima were no less clearly marked.

11 nous a paru convenable de faire quelques essais pour fixer les relations qui existent entre les rayons d une longueur d’onde aussi considerable et ceux qui sont emis par les sources franchement obscures, par exemple une lame de cuivre noircie et cbaullee a 3oo° ou meme a i5o°. Dans ce but nous avons fait les experiences suivantes :

Having formed a spectrum using a refracting apparatus made entirely of salt and a Bourbouze lamp as the source, we studied it from the perspective of its thermal distribution

Un spectre etant forme avec un appareil refringent tout en sel et la lampe Bourbouze comme source, nous l’avons etudie au point de vue de la distribution calorifique.

We thought it appropriate to conduct some experiments to determine the relationship between rays of such a long wavelength and those emitted by sources that are clearly dark, such as a copper plate blackened and annealed at 300°C or even at 150°C. To this end, we carried out the following experiments:

Au rouge extreme Faction galvanometrique etait 4°°7 aL1 maximum 0800, etc. Ges determinations faites, au platine incandescent nous avons substitue une lame de cuivre cbaullee a 3oo°. En observant alors les indications de notre tbermoscope, nous avons constate qu’elles etaient nulles tant que la distance de la pile a la position qu’elle occupait quand elle recevait les rayons d une flamme sodique n’atteignait pas i°; a partir de ce moment, lorsqu’on avancait vers la region de moindre ref rangi bill te, les eflets thermiques marchaient rapidement vers un maximum pour decroitre plus lentement ensuite. Ea position de la pile au moment de Faction maximum a ete prise par nous comme defimssant ce que l’on pourrait appeler l’indice moyen, ou plutot l’indice des rayons de plus grande efficacite de la lame.

At the extreme red end, the galvanometer reading was 400 at a maximum 5800, etc. Once these measurements were taken, we replaced the incandescent platinum with a copper plate heated to 300°. Upon observing the readings on our thermoscope, we found that they were zero as long as the distance from the pile to the position it occupied when receiving the rays of a sodium flame did not reach 1°; from that point on, as we moved toward the region of lower refractive index, the thermal effects rapidly increased toward a maximum and then decreased more slowly. We took the position of the cell at the moment of maximum intensity as defining what might be called the average index, or rather the index of the blade’s most effective rays.

En retablissant alors le spectre primitif, c’est-a-dire en remettant le platine incandescent a la place de la lame de cuivre, on determinait la longueur d'onde des rayons correspondant a cet indice moyen, et on la prenait pour longueur d’onde moyenne des rayons emis par la source obscure.

By restoring the original setup—that is, by replacing the copper plate with the incandescent platinum—the wavelength of the rays corresponding to this average index was determined, and this was taken as the average wavelength of the rays emitted by the dark source.

Nous avons cherche a controler l’exactitude des resultats que nous venous de faire connaitre et nous y sommes arrives en employant comme reseaux des echantillons de toiles metalliques du commerce. Ges toiles sont plus ou moins serrees, mais en general elles sont bien regulieres et, dans la lumiere homogene, elles donnent avec beaucoup de nettete et d eclat les plienomenes des franges successives. En employant des toiles de numeros difFerents, nous sommes toujours arrives aux memes longueurs d’onde pour des rayons de meme indice.

We sought to verify the accuracy of the results we had just reported, and we succeeded by using commercially available metal mesh samples as grids. These meshes vary in density, but in general they are quite uniform, and under uniform lighting, they clearly and vividly display the phenomena of successive fringes. By using meshes of different numbers, we consistently obtained the same wavelengths for rays of the same index.

Enfin, dans les regions voisines du maximum, nous avons constate que les resultats de nos observations s’accordent d’une maniere satisfaisante avec ceux que I’etude de cette meme region avait fournis a M. Mouton.

Le Tableau suivant resume l’ensemble de nos recherches.

Finally, in the regions adjacent to the maximum, we have found that the results of our observations are in satisfactory agreement with those obtained by Mr. Mouton in his study of the same region.

The following table summarizes the results of our research.

Dans la premiere colonne sont simplement transcrites les divi¬ sions de la regie le long de laquelle se mouvait la pile; dans la deuxieme, la distance angulaire qui separait les rayons etudies de ceux de la flamme sodique; dans les troisieme, quatrieme et cinquieme, les intensites qui correspondaient a ces rayons quand on employait comme source la lampe a platine incandescent, la plaque a 3oo°, la plaque a i5o°; dans la sixieme, les longueurs d’onde. Les nombres inscrits aux troisieme, quatrieme et cinquieme colonnes ont ete obtenus avec des appareils de sensibilites differentes soigneusement compares. Ils sont rapportes a une meme unite.

The first column simply lists the divisions of the scale along which the pile moved; in the second, the angular distance separating the studied rays from those of the sodium flame; in the third, fourth, and fifth, the intensities corresponding to these rays when using as a source the incandescent platinum lamp, the 300° plate, and the 150° plate; in the sixth, the wavelengths. The numbers listed in the third, fourth, and fifth columns were obtained using carefully compared instruments of different sensitivities. They are expressed in the same unit.

Divisions de la regie. Divisions of the scale.

Distance angulaire aux rayons du sodium. Angular distance from the sodium lines.

Lampe a platine incandescent. Incandescent platinum lamp.

Cuivre noir a 3oo°. Black copper at 300°.

Cuivre a 15o°. Copper at 150°.

Longueurs d’onde. Wavelengths.

SUMMARY:

Pierre Curie asks the question, when something is cooler, what kind of radiation does it emit?

Existing techniques for measuring infrared wavelengths at the time wouldn’t work at low temperatures, so he introducing a diffraction-based method that should.

He describes how he set up an experiment to measure invisible heat radiation (infrared). He shines the heat through a narrow opening and then through a finely spaced metal grid, which spreads the radiation out into a pattern—similar to how a prism spreads light into a rainbow. Because ordinary glass blocks this kind of radiation, he uses a special lens made of rock salt to focus it. He then moves a sensitive heat detector side to side to measure where the signal is strongest and weakest. This creates a repeating pattern of peaks, and by measuring the spacing between them, he can figure out the wavelength of the heat radiation. He also notes that making the opening wider or narrower only changes how strong the signal is, not where the peaks appear, which shows the pattern depends on the grid itself.

Pierre Curie explains that to get meaningful results, he can’t just measure a mix of heat rays—he needs to isolate one specific wavelength at a time and know exactly where it sits in the spectrum. To do this, he first creates a full spectrum of heat radiation using a very hot platinum light source and a prism made of rock salt (since normal glass would block the infrared). He then uses a movable slit to pick out a narrow slice of that spectrum—essentially selecting a single “color” of invisible heat. By carefully tracking the slit’s position relative to a known reference (like sodium light), he knows exactly which wavelength he’s working with.

He also notes a key challenge: as you move farther away from the visible part of the spectrum (toward deeper infrared), the radiation becomes much weaker and harder to detect. Even so, with careful adjustments—like widening the slit slightly—he’s still able to measure these longer wavelengths. His results show that he can successfully detect and measure heat radiation well beyond the range where ordinary materials like glass would block it.

Pierre Curie shifts from setting up the method to actually comparing different heat sources. He first maps out how heat radiation is distributed across the spectrum using a very hot platinum source. Then he asks an important question: how does this compare to objects that aren’t glowing—like a heated metal plate at 300°C or even 150°C?

When he replaces the bright platinum source with these cooler, “dark” objects, he finds that they don’t emit much radiation near the visible range at all. Instead, their heat shows up farther out in the infrared. As he moves his detector across the spectrum, the signal suddenly rises to a peak in this deeper, invisible region, then gradually fades. He treats this peak as representing the “average” wavelength of heat emitted by that object.

To figure out what that wavelength actually is, he switches back to the original setup with the platinum source and measures the same position more precisely using his diffraction method. This lets him assign a numerical wavelength to the radiation coming from the cooler object.

He double-checks his results using different metal grids and finds consistent answers, and he also notes that his measurements agree with earlier work by Mouton in overlapping regions. He finishes by presenting a table that ties everything together: positions in the spectrum, their distance from a known reference (sodium light), the intensity of heat from different sources, and the corresponding wavelengths.

Data Summary: The data show how heat radiation changes with temperature and position in the spectrum. Pierre Curie compares a very hot platinum source with copper heated to 300°C and 150°C, tracking both intensity and corresponding wavelengths across the spectrum. The hot source produces strong radiation over a wide range, including shorter wavelengths, while the cooler copper emits much weaker radiation that is concentrated at longer wavelengths. As temperature decreases, the peak of detectable radiation shifts steadily toward the infrared region and becomes less intense overall.