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Jackwork, Caurousel :
Jackwork comprising one or more horizontally rotating wheels or platforms carrying puppets which circulate around a vertical axis. They are used for displaying written time announcements in medieval Chinese astronomical clock towers and decoratively in later western public clocks.

Japanese Bell Clock :
Japanese bell clocks are spring-driven lantern movements in a temple-bell case. Balance wheels and fully developed rotating dials indicate that these clocks are of the late period, after the middle of the 18th century. The mechanism is inside the bell and the dial in its mouth. Such clocks were hard to read, as they sat on legs going through the dial. It may be, however, that the striking of the temple bell was the over-riding purpose of these clocks and that the dial only served to activate the strike and adjust the length of the hours for the season.

Japanning :
A pseudo-oriental form of decoration simulating Chinese or Japanese lacquer work. Japanning was extensively employed for decorating longcase clocks throughout much of the 18th century. Black with low-relief gilded decoration was the most popular form, but other colours like red and blue were often used. The drawback of japanned work was that in time the surface became covered with fine cracks, like an old painting.

Jewelling :
One of the early improvements in clockmaking was the replacement of brass pivot holes with pierced jewels. Precision timekeepers were jewelled with natural ruby or sapphire; in work of lower quality, garnet was used. The pallets of regulators are often jewelled with agate, but synthetic ruby is now used for most horological jewelling. Manufactured by automatic machines, it permits jewelling of movements for a fraction of the cost of former times. The high-speed escapement parts should receive attention from the jeweller before the slower-moving train wheels, and in the lever escapement, the impulse pin, the pallet stones, the end stones and pivot holes of the balance are the seven most important jewels.

Jockele Clock :
The usual size for the movement of a Black Forest clock is about 6 1/4in. high, 4 1/4in. broad and 5 1/2in. deep. About 1760 a smaller version was made nicknamed Schottenuhr, and about 1790 Jakob Herbstreith of Hinterzarten near Neustadt produced the Jockele which is even smaller, measuring about 3 1/4in. high, 2 3/8in. broad and 1 1/4in. deep.

Jumper :
A spring-loaded arm, a type of click, shaped to hold the teeth of a star wheel stationary between movements, although allowing the wheel to rotate in either direction.