What was that device which Sherlock Holmes used as a weapon on Dredger?
It is not a weapon, it was a circuit connector, however, presented as if it was an electric capacitor charged by a static electricity generator (we can see Sherlock charging it by spinning the wheel).
A capacitor is a device that stores electrical energy in an electric field. It is a passive electronic component with two terminals. [...] Most capacitors contain at least two electrical conductors often in the form of metallic plates or surfaces separated by a dielectric medium.
I am sure that the producers were inspired by devices such as this one:
However, in the picture above the "jar" is the capacitor and the "stick" is used to discharge the electricity by short-circuiting it.
The device in the movie, however, wouldn't work as displayed - if you look closely, you will see that the two prongs are joined together (and probably made of copper) which means that the even if the handle was somehow holding the electric charge, it would immediately be discharged by this connection instead of waiting for the contact with the skin.
On the other hand, it could be a wood (an electric insulator) just painted to look like copper and only the tips of the "fork" are connected to the capacitor in the handle...
The script describes it as a 'short shaft', basically a piece of metal connected to the electrical device on the desk.
INT. REORDON'S LAB
Dredger bears down on Holmes. Holmes reaches out for some kind of weapon, anything he can grab. He snatches a SHORT SHAFT. A truly pathetic weapon.
Dredger lurches toward Holmes, who swings the metal shaft desperately, and... the shaft SPARKS.
ZAP! A flash of light and crackle, and Dredger gets an ELECTRIC SHOCK that sends him flying back twenty feet --
INT. REORDON'S ROOMS
Watson finishes off his thug and plucks the engagement ring off the floor, just as Dredger flies through the air, SLAMMING into Watson, knocking him off his feet, and knocking the ring away.
Holmes looks at the lightning rod in his hand, stunned by the efficacy of the weapon.
Dredger tries to recover, climbing off of Watson. Holmes cranks his weapon. The rod sparks and hums.
Dredger pulls himself up with the help of a water pipe on the wall. Holmes thinks quickly, then touches the other end of the pipe with the rod, and ZZZAPPPP!! The electrical current RUNS ALONG THE PIPE, and KNOCKS Dredger back twenty more feet.