A D7 class battle cruiser firing a magnetic pulse
A magnetic pulse was a type of weapon deployed by Klingon D7 class battle cruisers during the mid-2260s.
On stardate 3198, a Klingon attack ship fired magnetic pulses at the USS Enterprise in an unprovoked surprise attack at the outbreak of the first Klingon-Federation War. The Enterprise returned phaser fire, destroying the warship, but not before receiving blast damage to decks 10 and 11, minor buckling in the antimatter pods, and minor casualties. (TOS: "Errand of Mercy")
Background
Original shot of the Enterprise hit by magnetic pulses
In the original version of "Errand of Mercy", the effect of Enterprise being hit by energy was a reuse of footage originally taken from TOS: "Arena". In the 2007 remastering of "Errand of Mercy", the weapon was shown being fired from or near the torpedo launcher located on the bow of the ship, indicating that this is perhaps a variation in their torpedo settings, in the vein of the Enterprise's proximity blast setting, or another altogether unique weapon design.
The concept behind the magnetic pulse effect could explain why the shot fired at the Enterprise and the Grissom, from Kruge's Bird-of-Prey, in Star Trek III was an electric arc rather than the explosion that a torpedo might cause.