A few minutes into the Opus 8 Trio of Brahms, cellist Frans Helmerson stopped playing last night at the Library of Congress, held up his bow and looked at it quizzically while his companions, violinist Arve Tellefsen and pianist Ralf Gothoni, ground to a halt. "I'm terribly sorry," he said in flawless English, "my bow just broke. If you will just wait a minute . . ." and he hurried offstage.

It was hardly a wonder that the bow succumbed, considering the kind of energy that had been flowing through it. Helmerson is scheduled to perform the Dvorak Concerto in January under the baton of fellow cellist Mstislav Rostropovich, and it might be wise to reserve tickets early, before word gets around about the kind of electricity he generates. After he returned with another bow, the music was resumed and gradually climbed back to its former high voltage, but for a while it sounded a shade less intense and coordinated than before the accident. It was the least satisfactory performance of the evening, but it was brilliant.

The Nordic Trio (brought together for the "Scandinavia Today" festival) sounded like a well-seasoned ensemble last night -- not quite as telepathically together as the Beaux Arts Trio, perhaps, but as precise, technically skilled and immersed in the music as one has any right to expect of mere mortals.

The musicians received a standing ovation at the end, a very rare event in the Library of Congress and an honor that has to be well earned. It was particularly unexpected from last night's audience, which contained a high proportion of Washington's musical elite -- not the social elite, but the people who compose our music, perform it, broadcast it and manage our musical organizations. An encore was demanded and supplied: a trio movement by Niels Gade.

The audience heard, besides the interrupted Brahms, the world premiere of a fascinating trio by Thurkell Sigurbjo rnsson of Iceland and an incandescent performance of the Shostakovich Trio No. 2 in E minor. The players were dramatic and soulful in the first movement; feverishly brilliant in the scherzo-flavored second; sweetly elegiac in the largo and magnificently controlled in the finale, where intensity builds in a long, technically demanding crescendo. In this final movement, when they had raised the musical temperature as high as it could be raised through increments of speed and dynamics, the trio showed mastery of a special art -- tightening the tension one more notch by a slight slowing down, then gliding gracefully into the quiet conclusion. The effect was magnificent.

Sigurbjo rnsson's Trio has a title, "Three Faces of Pantomime," though the composer says that he had no particular pictorial or narrative element in mind when he composed it, merely the feeling that "there was a lot of pantomime in the music." There is indeed.

The music resembles that of Elliott Carter, perhaps, in the way it gives each of the instruments a rather clearly defined character and then engages them in a dialogue where the characters interact and undergo changes. But on first hearing, it seemed to stop before reaching the level of complexity at which Carter seems to feel most at home -- and no mention of other composers should overshadow the fact that the Trio is a strikingly original work.

It might be interpreted, in a sense, as a dialogue between styles -- a rather gruff modernism espoused by the piano, particularly in its lower notes, and a lyric romanticism embodied by the violin playing a flowing melody on its upper strings. The cello stands somewhere between these extremes, leaning toward the piano in a long pizzicato sequence at the beginning and later communing with the violin after the cellist begins to use his bow. During the interaction that follows, the piano tries the violin's approach tentatively, moves back toward its original attitude and experiments with other possibilities. The music ends in reconciliation, a blending of the various possibilities that have been explored, though perhaps a bit closer to the violin's original premise than to the piano's. There is undoubtedly much more in the music than can be detected in a single hearing, but it has the virtue (which seems to be returning in new music) of engaging the listener's attention and feelings right from the start. It may be a masterpiece; it is certainly eloquent and well crafted.

With Sigurbjo rnsson and Karolina Eriksdottir, whose music was heard Wednesday night at the National Academy of Sciences, Iceland has made a strong musical impression this week -- particularly remarkable for a country whose population is about the same size as that of Mobile, Ala.