# Who originally derived the general force law equation of force between current elements?

Wikipedia credits this to Maxwell. This derivation can be found in Maxwell's Treatise on Electricity and Magnetism vol. 2, part 4, ch. 2 (§§502-527). I went through the derivation and found two self cancelling mistakes made by Maxwell. Those mistakes were corrected by J.J. Thomson in his edition of Maxwell's treatise.

The fact that Maxwell made some mistakes and that the final result is unaffected is itself an indication that someone else before Maxwell originally made that derivation and Maxwell simply took this derivation from someone else (although he doesn't mention in his treatise from where he took this derivation).

So was it really Maxwell who originally made this derivation of general force law equation or was it someone else?

EDIT: (@Geremia)

Ampere derived in 1820s the force between current elements. He made an assumption that it was along the line joining the two elements. A few scientists after Ampere pointed out that current elements have direction as well and hence there is not a solid reason on why force between current elements would be like other forces in nature. In 1845, Hermann Grassmann derived another force law (which is taught today in schools). The force law of Grassmann doesn't obey Newton's third law even in the weak form.

In the year 1873, Maxwell published his treatise. In that treatise, there is a derivation (Vol 2, Article 502-527) of general force law equation. The general force law equation says there can be infinite valid force laws as far as source circuit is closed. The general force law equation is:

$$d^{2}\vec{F}=kII'dsds'\left[\left(\frac {1}{r^{2}}\left({\frac {\partial r}{\partial s}}{\frac {\partial r}{\partial s'}}-2r{\frac {\partial ^{2}r}{\partial s\partial s'}}\right) +r{\frac {\partial ^{2}Q}{\partial s\partial s'}}\right)(\hat{r})-{\frac {\partial Q}{\partial s'}}(\hat{s})+{\frac {\partial Q}{\partial s}}(\hat{s'})\right]$$

where:

$$\hat{r}$$ is a unit vector pointing from $$s$$ to $$s'$$ (field circuit to source circuit)

$$Q$$ is any function of $$r$$. But we must be careful while assuming the value of $$Q$$ because we should give same form for $$Q$$ in $$r$$ and $$s'$$ coordinates.

This general force law equation can be found in article 525 of Maxwell's treatise. Thus I am only asking who originally made this "general force law equation" (and not Ampere's original force law).

Wikipedia call's it Maxwell's 1873 derivation. As explained in my original post, it doesn't seem to me that it was Maxwell who originally wrote it down.

Please tell if it was really Maxwell or someone else?

• This is called Ampere's Force Law and the formula was derived by Ampere. Dec 30 '17 at 18:09
• Where does "Wikipedia credit this to Maxwell"? Dec 30 '17 at 21:20
• I don't think you've copied Maxwell's equation down correctly from Treatise vol. 2 p. 160. Ampère writes the general formula on p. 370 (PDF p. 384) of this translation of his 1826 Mémoire. Jan 4 '18 at 23:05
• I am talking about equation 38 page 160 in Maxwell's treatise. This equation is called General Force Law equation
– Joe
Jan 5 '18 at 4:50
• @Joe Yes, that's what I'm referring to, too. Jan 8 '18 at 20:46

Ampère did. Ampère's force law (not to be confused with one of Maxwell's equations, "Ampère"'s circuital law, which Ampère never wrote down, as Ampère didn't deal with the field concept), written in modern vector notation, gives the force that current elements $I_1 d\vec {\ell }_1$ and $I_2 d\vec {\ell }_2$ exert on one another to be:

$$d^2\vec{F_{21}^A} = - \frac{\mu _0 }{4\pi }I_1 I_2 \frac{\hat {r}_{12} }{r_{12}^2 }\left[2(d\vec {\ell }_1 \cdot d\vec {\ell }_2) - 3({\hat {r}_{12} \cdot d\vec {\ell }_1 })({\hat {r}_{12} \cdot d\vec {\ell }_2 })\right] = - d^2\vec{F_{12}^A}.$$

Here's the English translation of Ampère's work where he derives his force law:

• Your answer is good and indeed correct. However I am asking about general force law and not Ampere's force law. Ampere's for law is a special case of general force law
– Joe
Jan 2 '18 at 12:46
• You are mistaken a bit here. Ampere ASSUMES the force is along the line joining the elements. Ampere force law is not "general" (by general I mean an equation satisfying all force laws). Since it is not general, the force law of Grassmann (between two current elements) cannot be derived from that of Ampere. However general force law equation not only derives Ampere and Grassmann force laws but also shows all other valid force laws (compatible with closed circuits).
– Joe
Jan 2 '18 at 15:31
• However if the source circuit is closed, all force laws are same and then Grassmann force law can be derived from that of Ampere.
– Joe
Jan 2 '18 at 15:32
• According to Ampere, $n=2$ and not $\frac{1}{2}$
– Joe
Jan 2 '18 at 15:40
• @Joe Correction: Ampère wrote the most general form and used his experiments to determine $n=2$, $k=−1/2$, and $h=−3/2$ (cf. Assis & Chaib 2015 p. 27 // PDF p. 30). Jan 2 '18 at 21:23