Formula for evaluation of character on a transposition

Let $\lambda\vdash n$ be a partition of $n\in\mathbb N$ and $\chi=\chi_\lambda$ the corresponding irreducible character of the symmetric group $S_n$. Denote by $\lambda^t$ be the transpose of $\lambda$ and $\ell(\lambda):=\lambda^t_1$ the length of $\lambda$. Let $\tau\in S_n$ be any transposition. In [R, Theorem 3.5], the following formula is quoted: $$ \binom n2 \cdot \frac{\chi(\tau)}{\chi(1)} = \sum_{i=1}^{\ell(\lambda)} \left( \binom{\lambda_i}2 - \binom{\lambda^t_i}2 \right). $$

I would like to see a proof for this formula. Roichman refers to the paper [I], which itself refers to a German paper by Frobenius, "Über die Charaktere der symmetrischen Gruppe". I was unable to obtain a copy of said paper, but I believe this must somehow follow from the Murnaghan-Nakayama rule (or directly from the Frobenius character formula).

Frankly, I have not invested a lot of time into an attempt to prove this myself because I was hoping that there is a more accessible and detailed reference for such an early formula. I would be very grateful if someone could provide such reference. Of course, if you happen to know a short proof, don't refrain from posting it.

• [R] Yuval Roichman: Upper bound on the characters of the symmetric groups. Inventiones mathematicae, August 1996, Volume 125, Issue 3, pp. 451-485.
• [I] Ingram, R.E.: Some characters of the symmetric group. Proceedings of the American Mathematical Society, June 1950, Volume 1, Issue 3, pp 358-369.

The paper Über die Charaktere der symmetrischen Gruppe by Frobenius is in the Sitzungsberichte der königlich preußischen Akademie der Wissenschaften zu Berlin of $1900$, available at archive.org in various formats. It begins on p. $516$; the formula you cite seems to correspond to the last formula of the paper on p. $534$. The notation $f^{(\kappa)}=\chi^{(\kappa)}_0$ is introduced on p. $522$, and $h_\varrho=n!/(c(n-c)!)$ is introduced on p. $533$. I'm not sure though how the $a_i$ and $b_i$ are related to your $\lambda_i$ and $\lambda_i^t$. Let me know if you need any help with the German.