Bianchi identity satisfied by the gauge invariant Ramond-Ramond four-form field strength $\tilde F_4 = dC_3 + A_1 \wedge H_3$ of which the D4-brane is a magnetic source. We have the relation $$\label{mbi}
d\tilde F_4 + F_2 \wedge H_3 = *j^{bs}_{D4},$$ where the right hand side is the brane-source current (which vanishes in the absence of an explicit D4-brane source). Here, $F_2$ is the usual IIA Ramond-Ramond two-form field strength and $H_3 = dB_2$ is the Neveu-Schwarz field strength. Taking an exterior derivative of (\[mbi\]) shows that $d*j^{bs}_{D4}$ does not vanish. Instead, a flux of $F_2$ falling on an NS5 brane (where $*j_{NS5}^{bs} \equiv
dH_3 \neq 0$) or a flux of $H_3$ falling on a D6-brane (where $*j^{bs}_{D6} \equiv dF_2 \neq 0$) acts as a source or sink of D4-brane charge. Some of the subtleties of defining charge and working with brane-source currents are discussed in [@3Q], but it is enough for us that this result leads to the Hanany-Witten effect and the associated creation of a D4-brane as described below.

The diagram below shows various stages in this process for the case of an NS5-brane moving past a D6-brane to make a D4-brane [@GM]. Similar results also follow for D$p$ 